# Professional services firms headquartered in London

Tamtam tracks 1,248 professional services firms headquartered in London. This list ranks the 30 largest by number of employees on LinkedIn, led by EY, G4S and Aon. Their median headcount is 13,502. Half of them were founded in 1987 or later.

These London-based professional services companies are mostly large, established firms. Among them are EY, a global organization providing assurance, consulting, and tax services, and Aon, which offers advice and solutions to clients worldwide. Other notable companies include Hays, a specialist recruitment and workforce solutions provider, and Arup, a global collective of designers and consultants. Many of these companies, such as G4S (founded 1901) and Smollan (founded 1931), have long histories.

The list reveals a concentration of companies in various professional services sectors, including financial services, consulting, and engineering. For example, Aon and WTW both operate in financial services, while EY-Parthenon and KPMG UK specialize in business consulting. This indicates a diverse yet interconnected professional services landscape within London.

🇬🇧 London · 30 companies · Updated Nov 12, 2026

| # | Company | Industry | Headquarters | Employees | Founded |
|---|---|---|---|---|---|
| 1 | [EY](https://www.tamtam.ai/c/ey) (ey.com)<br>A global organization with diverse teams in over 150 countries provides assurance, consulting, law, strategy, tax, and transactions to help clients grow and transform | Professional Services | 🇬🇧 London | 404,207 |  |
| 2 | [G4S](https://www.tamtam.ai/c/g4s) (g4s.com)<br>A leading security and facility services company providing proactive security services, cutting-edge technology, and integrated solutions to help clients focus on their core business | Security and Investigations | 🇬🇧 London | 116,486 | 1901 |
| 3 | [Aon](https://www.tamtam.ai/c/aon) (aon.com)<br>The company provides advice and solutions to clients worldwide, helping them make better decisions to protect and grow their business | Financial Services | 🇬🇧 London | 93,574 |  |
| 4 | [WTW](https://www.tamtam.ai/c/wtw) (wtwco.com)<br>Offering data-driven solutions in people, risk, and capital areas, leveraging global expertise to enhance strategy, resilience, workforce motivation, and performance | Financial Services | 🇬🇧 London | 55,656 |  |
| 5 | [Hays](https://www.tamtam.ai/c/hays) (haysplc.com)<br>Specialist recruitment and workforce solutions provider offering advisory services, career transitions, and employer brand positioning, with a focus on empowering people and businesses to succeed globally | Staffing and Recruiting | 🇬🇧 London | 31,854 |  |
| 6 | [Arup](https://www.tamtam.ai/c/arup) (arup.com)<br>A global collective of designers and consultants dedicated to sustainable development, collaborating with clients to shape a better world using technology and imagination | Design Services | 🇬🇧 London | 26,193 | 1946 |
| 7 | [Capita](https://www.tamtam.ai/c/capita) (capita.com)<br>A leading provider of business services using data, technology, and people to simplify connections between governments, citizens, businesses, and customers | IT Services and IT Consulting | 🇬🇧 London | 25,581 | 1984 |
| 8 | [Kantar](https://www.tamtam.ai/c/kantar) (kantar.com)<br>Global leader in marketing data and analytics with deep expertise, innovative technology, and insights to help clients understand people and drive growth worldwide | Market Research | 🇬🇧 London | 24,976 |  |
| 9 | [Mott MacDonald](https://www.tamtam.ai/c/mott-macdonald) (mottmac.com)<br>The company focuses on improving society through digital innovation, excellence, and solving intricate challenges with clients to deliver better outcomes | Civil Engineering | 🇬🇧 London | 24,653 | 1989 |
| 10 | [EY-Parthenon](https://www.tamtam.ai/c/ey-parthenon) (ey.com)<br>Multidisciplinary professionals across 120 countries provide actionable corporate, transaction, and turnaround strategies to clients using data analytics and digital capabilities | Business Consulting and Services | 🇬🇧 London | 18,536 |  |
| 11 | [KPMG UK](https://www.tamtam.ai/c/kpmg-uk) (kpmg.com)<br>A leading UK provider of tax, audit, and advisory services with a global presence, focusing on innovative solutions and expertise for clients' needs | Business Consulting and Services | 🇬🇧 London | 15,102 | 1987 |
| 12 | [Petrofac](https://www.tamtam.ai/c/petrofac) (petrofac.com)<br>International energy service provider specializing in designing, building, managing, and maintaining oil, gas, refining, and renewable energy infrastructure for leading energy companies worldwide | Engineering Services | 🇬🇧 London | 14,616 | 1981 |
| 13 | [Cloud Consulting](https://www.tamtam.ai/c/cloud-consulting) (ccltng.com)<br>Oversees national education policies, youth programs, and administration to support learning and development across France. | IT Services and IT Consulting | 🇬🇧 London | 14,588 |  |
| 14 | [Savills](https://www.tamtam.ai/c/savills) (savills.com)<br>Global real estate advisory firm providing tailored solutions and expert advice for residential and commercial property clients worldwide. | Real Estate | 🇬🇧 London | 13,891 |  |
| 15 | [Smollan](https://www.tamtam.ai/c/smollan) (smollan.com)<br>Partners with brands and retailers, building solutions that drive growth and create opportunities at the point of purchase. | Retail | 🇬🇧 London | 13,502 | 1931 |
| 16 | [Amey](https://www.tamtam.ai/c/amey) (amey.com)<br>A leading provider of engineering, operations, and decarbonisation solutions for transport infrastructure and complex facilities, focusing on sustainability and resilience | Civil Engineering | 🇬🇧 London | 11,000 |  |
| 17 | [PwC UK](https://www.tamtam.ai/c/pwc-uk) (pwc.co.uk)<br>Professional services firm focusing on building trust in society and solving important problems through partnership and commercial expertise. | Accounting | 🇬🇧 London | 10,873 |  |
| 18 | [BSI](https://www.tamtam.ai/c/bsi-london) (bsigroup.com)<br>A global organization with over 100 years of experience in shaping best practices, helping businesses achieve excellence, competence, and sustainable growth | Professional Services | 🇬🇧 London | 9,882 | 1901 |
| 19 | [Mace](https://www.tamtam.ai/c/mace) (macegroup.com)<br>Delivery consultancy and construction expertise for iconic buildings, data centres, life science, social infrastructure, and regeneration projects worldwide. | Construction | 🇬🇧 London | 9,872 | 1990 |
| 20 | [Endava](https://www.tamtam.ai/c/endava) (endava.com)<br>The company leverages technology and expertise to drive innovation and transform businesses with customized digital solutions across industries | IT Services and IT Consulting | 🇬🇧 London | 9,559 | 2020 |
| 21 | [Apex Group Ltd (UK & Ireland)](https://www.tamtam.ai/c/apex-group-ltd-uk-and-ireland) (apexgroup.com)<br>A global financial services provider with offices in over 50 countries, offering a wide range of services to asset managers, capital markets, and family offices | Financial Services | 🇬🇧 London | 9,281 | 2003 |
| 22 | [AMS](https://www.tamtam.ai/c/ams) (weareams.com)<br>Global talent solutions business specializing in sourcing, selecting, and retaining top talent through digital innovation, HR expertise, and a holistic approach to workforce management | Human Resources Services | 🇬🇧 London | 8,718 | 1996 |
| 23 | [Supporting Education Group](https://www.tamtam.ai/c/supporting-education-group) (supportingeducation.com)<br>Providing qualified teachers, support staff, professional development, and tailored services to improve education quality for schools and disadvantaged students. | Education | 🇬🇧 London | 8,571 |  |
| 24 | [Clifford Chance](https://www.tamtam.ai/c/clifford-chance) (cliffordchance.com)<br>A global law firm providing high-quality legal advice to a diverse range of clients, including corporations, governments, regulators, and non-profit organizations worldwide | Law Practice | 🇬🇧 London | 8,435 | 1987 |
| 25 | [Capco](https://www.tamtam.ai/c/capco) (capco.com)<br>Global technology and management consultancy specializing in digital transformation, data-driven solutions, and innovation for banking, payments, capital markets, insurance, and energy sectors. | Financial Services | 🇬🇧 London | 8,026 | 1998 |
| 26 | [SThree](https://www.tamtam.ai/c/sthree) (sthree.com)<br>Connecting skilled STEM professionals with organizations to drive innovation and solve complex challenges through specialist brands and tailored workforce solutions. | Business Consulting and Services | 🇬🇧 London | 7,927 |  |
| 27 | [Acuity Analytics](https://www.tamtam.ai/c/acuity-analytics) (acuityanalytics.com)<br>Provides bespoke research, analytics, and technology solutions to financial services, helping institutions operate efficiently and unlock human capital to drive revenue. | Financial Services | 🇬🇧 London | 7,619 | 2003 |
| 28 | [Azets](https://www.tamtam.ai/c/azets) (azets.com)<br>A leading professional services firm in Northern Europe, providing advisory, accounting, audit, HR, recruitment, outsourcing, and technology solutions to clients | Accounting | 🇬🇧 London | 7,523 |  |
| 29 | [Clyde & Co](https://www.tamtam.ai/c/clyde-and-co) (clydeco.com)<br>A global law firm specializing in insurance, transport, infrastructure, energy, and trade Committed to diversity, community support, and environmental sustainability | Law Practice | 🇬🇧 London | 7,472 | 1933 |
| 30 | [Linklaters](https://www.tamtam.ai/c/linklaters) (linklaters.com)<br>It is a global law firm providing legal expertise and innovative solutions to clients navigating complex markets and regulatory environments. | Law Practice | 🇬🇧 London | 7,110 |  |

## How this list is built

Every company on this list has a Tamtam company profile. The industry is the one the company gives on LinkedIn; categories such as SaaS or fintech are classified by Tamtam's AI from the company's website and LinkedIn page; and the headquarters city must match the company's own address. Companies are ranked by the number of employees on LinkedIn, and the list is refreshed every night. Classifications can be wrong, so treat the list as a starting point rather than an official ranking. Last updated Nov 12, 2026.

## More company lists in London

- [The largest companies in London](https://www.tamtam.ai/l/largest-companies-london) (12,882)
- [Multinational companies in London](https://www.tamtam.ai/l/multinational-companies-london) (3,652)
- [Financial Services companies in London](https://www.tamtam.ai/l/financial-services-companies-london) (1,862)
- [Financial services firms in London](https://www.tamtam.ai/l/financial-services-firms-london) (1,824)
- [SaaS companies in London](https://www.tamtam.ai/l/saas-companies-london) (1,615)
- [Software Development companies in London](https://www.tamtam.ai/l/software-development-companies-london) (1,217)
- [Retail chains in London](https://www.tamtam.ai/l/retail-chains-london) (942)
- [IT Services and IT Consulting companies in London](https://www.tamtam.ai/l/it-services-and-it-consulting-companies-london) (693)
- [Companies with a mobile app in London](https://www.tamtam.ai/l/companies-with-mobile-app-london) (667)
- [Consulting firms in London](https://www.tamtam.ai/l/consulting-firms-london) (648)
- [IT services companies in London](https://www.tamtam.ai/l/it-services-companies-london) (607)
- [Subscription businesses in London](https://www.tamtam.ai/l/subscription-businesses-london) (544)
- [Venture capital and private equity firms in London](https://www.tamtam.ai/l/vc-and-private-equity-firms-london) (539)
- [Marketing and communications agencies in London](https://www.tamtam.ai/l/marketing-agencies-london) (533)
- [Retail Apparel and Fashion companies in London](https://www.tamtam.ai/l/retail-apparel-and-fashion-companies-london) (498)
- [Advertising Services companies in London](https://www.tamtam.ai/l/advertising-services-companies-london) (480)
- [Companies with in-house R&D in London](https://www.tamtam.ai/l/r-and-d-companies-london) (477)
- [AI companies in London](https://www.tamtam.ai/l/ai-companies-london) (456)
- [Investment Management companies in London](https://www.tamtam.ai/l/investment-management-companies-london) (452)
- [Hotel and hospitality groups in London](https://www.tamtam.ai/l/hospitality-groups-london) (445)
- [Luxury brands in London](https://www.tamtam.ai/l/luxury-brands-london) (428)
- [AI-native companies in London](https://www.tamtam.ai/l/ai-native-companies-london) (407)
- [E-commerce companies in London](https://www.tamtam.ai/l/ecommerce-companies-london) (384)
- [Fintech companies in London](https://www.tamtam.ai/l/fintech-companies-london) (384)

## The same list elsewhere in United Kingdom

- [Professional services firms in the United Kingdom](https://www.tamtam.ai/l/professional-services-firms-united-kingdom) (2,706)
- [Professional services firms in Scotland](https://www.tamtam.ai/l/professional-services-firms-scotland) (112)
- [Professional services firms in Manchester](https://www.tamtam.ai/l/professional-services-firms-manchester) (86)
- [Professional services firms in Leeds](https://www.tamtam.ai/l/professional-services-firms-leeds) (55)
- [Professional services firms in Edinburgh](https://www.tamtam.ai/l/professional-services-firms-edinburgh) (43)
- [Professional services firms in Birmingham](https://www.tamtam.ai/l/professional-services-firms-birmingham) (37)
- [Professional services firms in Glasgow](https://www.tamtam.ai/l/professional-services-firms-glasgow) (29)
- [Professional services firms in Bristol](https://www.tamtam.ai/l/professional-services-firms-bristol) (25)

## The same list elsewhere

- [Professional services firms in the world](https://www.tamtam.ai/l/professional-services-firms-in-the-world) (24,737)
- [Professional services firms in Europe](https://www.tamtam.ai/l/professional-services-firms-in-europe) (13,698)

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Source: https://www.tamtam.ai/l/professional-services-firms-london