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.
| # | Company | Employees | |||
|---|---|---|---|---|---|
| 1 | 404,207 | ||||
| 2 | 116,486 | ||||
| 3 | 93,574 | ||||
| 4 | 55,656 | ||||
| 5 |
Hayshaysplc.com
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 |
31,854 | |||
| 6 | 26,193 | ||||
| 7 |
Capitacapita.com
A leading provider of business services using data, technology, and people to simplify connections between governments, citizens, businesses, and customers |
25,581 | |||
| 8 |
Kantarkantar.com
Global leader in marketing data and analytics with deep expertise, innovative technology, and insights to help clients understand people and drive growth worldwide |
24,976 | |||
| 9 |
Mott MacDonaldmottmac.com
The company focuses on improving society through digital innovation, excellence, and solving intricate challenges with clients to deliver better outcomes |
24,653 | |||
| 10 |
EY-Parthenoney.com
Multidisciplinary professionals across 120 countries provide actionable corporate, transaction, and turnaround strategies to clients using data analytics and digital capabilities |
18,536 | |||
| 11 | 15,102 | ||||
| 12 |
Petrofacpetrofac.com
International energy service provider specializing in designing, building, managing, and maintaining oil, gas, refining, and renewable energy infrastructure for leading energy companies worldwide |
14,616 | |||
| 13 | 
Cloud Consultingccltng.com
Oversees national education policies, youth programs, and administration to support learning and development across France. |
14,588 | |||
| 14 |
Savillssavills.com
Global real estate advisory firm providing tailored solutions and expert advice for residential and commercial property clients worldwide. |
13,891 | |||
| 15 |
Smollansmollan.com
Partners with brands and retailers, building solutions that drive growth and create opportunities at the point of purchase. |
13,502 | |||
Want more companies than this list?This page shows the largest ones only. Connect to Tamtam to see every company that matches, filter by size, industry or growth, and reach their decision-makers. | |||||
| 16 | 11,000 | ||||
| 17 | 10,873 | ||||
| 18 |
BSIbsigroup.com
A global organization with over 100 years of experience in shaping best practices, helping businesses achieve excellence, competence, and sustainable growth |
9,882 | |||
| 19 |
Macemacegroup.com
Delivery consultancy and construction expertise for iconic buildings, data centres, life science, social infrastructure, and regeneration projects worldwide. |
9,872 | |||
| 20 |
Endavaendava.com
The company leverages technology and expertise to drive innovation and transform businesses with customized digital solutions across industries |
9,559 | |||
| 21 |
Apex Group Ltd (UK & Ireland)apexgroup.com
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 |
9,281 | |||
| 22 |
AMSweareams.com
Global talent solutions business specializing in sourcing, selecting, and retaining top talent through digital innovation, HR expertise, and a holistic approach to workforce management |
8,718 | |||
| 23 |
Supporting Education Groupsupportingeducation.com
Providing qualified teachers, support staff, professional development, and tailored services to improve education quality for schools and disadvantaged students. |
8,571 | |||
| 24 |
Clifford Chancecliffordchance.com
A global law firm providing high-quality legal advice to a diverse range of clients, including corporations, governments, regulators, and non-profit organizations worldwide |
8,435 | |||
| 25 | 8,026 | ||||
| 26 |
SThreesthree.com
Connecting skilled STEM professionals with organizations to drive innovation and solve complex challenges through specialist brands and tailored workforce solutions. |
7,927 | |||
| 27 |
Acuity Analyticsacuityanalytics.com
Provides bespoke research, analytics, and technology solutions to financial services, helping institutions operate efficiently and unlock human capital to drive revenue. |
7,619 | |||
| 28 | 7,523 | ||||
| 29 |
Clyde & Coclydeco.com
A global law firm specializing in insurance, transport, infrastructure, energy, and trade Committed to diversity, community support, and environmental sustainability |
7,472 | |||
| 30 |
Linklaterslinklaters.com
It is a global law firm providing legal expertise and innovative solutions to clients navigating complex markets and regulatory environments. |
7,110 | |||
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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 12,882
- Multinational companies in London 3,652
- Financial Services companies in London 1,862
- Financial services firms in London 1,824
- SaaS companies in London 1,615
- Software Development companies in London 1,217
- Retail chains in London 942
- IT Services and IT Consulting companies in London 693
- Companies with a mobile app in London 667
- Consulting firms in London 648
- IT services companies in London 607
- Subscription businesses in London 544
- Venture capital and private equity firms in London 539
- Marketing and communications agencies in London 533
- Retail Apparel and Fashion companies in London 498
- Advertising Services companies in London 480
- Companies with in-house R&D in London 477
- AI companies in London 456
- Investment Management companies in London 452
- Hotel and hospitality groups in London 445
- Luxury brands in London 428
- AI-native companies in London 407
- E-commerce companies in London 384
- Fintech companies in London 384
The same list elsewhere in United Kingdom
- Professional services firms in the United Kingdom 2,706
- Professional services firms in Scotland 112
- Professional services firms in Manchester 86
- Professional services firms in Leeds 55
- Professional services firms in Edinburgh 43
- Professional services firms in Birmingham 37
- Professional services firms in Glasgow 29
- Professional services firms in Bristol 25