These London-based companies specialize in cybersecurity, telecommunications, and IT services, representing a mix of large established corporations and mid-sized firms. Notable examples include BT Group, a global communications services company; BAE Systems, providing advanced technology-led defense and security solutions; Vodafone Business, offering mobile, fixed, and security services; and Noventiq, a digital transformation and cybersecurity provider.
The data shows a strong concentration of companies founded in the early 2000s, with Claranet (1996), BAE Systems (1999), Gamma (2002), and Mimecast (2003) illustrating this trend. This suggests a period of significant growth and development in these sectors within London.
| # | Company | Employees | |||
|---|---|---|---|---|---|
| 1 | 73,712 | ||||
| 2 |
BAE Systemsbaesystems.com
The company provides advanced technology-led defense, aerospace, and security solutions, with a skilled workforce in over 40 countries, focusing on innovation and long-term commitment |
55,143 | |||
| 3 |
Vodafone Businessvodafone.com
Providing mobile, fixed, unified communications, IoT, cloud, and security services to businesses globally, regionally, and locally. |
8,100 | |||
| 4 |
Noventiqnoventiq.com
A global digital transformation and cybersecurity solutions provider connecting 80,000+ organizations, with a diverse portfolio and active M&A strategy for growth |
3,941 | |||
| 5 |
TryHackMetryhackme.com
It simplifies learning and teaching Cybersecurity through prebuilt courses and cloud-hosted virtual machines, ideal for CTFs, workshops, and training. |
3,445 | |||
| 6 |
Claranetclaranet.com
A global technology services provider offering managed services, professional services, and training in Cloud, Networks, Cyber Security, and Workplace technologies since 1996 |
3,073 | |||
| 7 |
Quantum Security Agencyorg.uk
Specialized restaurant marketing agency offering solutions to help restaurants thrive through social media management, content marketing, websites, and other marketing services |
2,842 | |||
| 8 |
Gammagammagroup.co
Gamma is a leading supplier of Unified Communications services in the UK, Germany, Spain, and Benelux, offering robust solutions for communication and collaboration |
2,588 | |||
| 9 |
Mimecastmimecast.com
Protects humans, data, and AI agents through a unified platform, securing the workforce and their interactions with AI from cyberattacks. |
2,396 | |||
| 10 |
Cybersecurityorg.uk
Cybersecurity experts providing incident response training, social engineering defense, vulnerability scanning, threat hunting, API testing, mobile app security, privacy compliance, user awareness training, and more |
1,209 | |||
| 11 |
Maintelmaintel.co.uk
Provider of cloud communications and managed services, specializing in digital transformation for public and private sectors Delivering digital workplace, customer experience, and secure connectivity solutions globally |
946 | |||
| 12 |
GCXgcxworld.com
Network services enabling digital transformation through managed SD-WAN, hybrid networks, and extensive subsea cloud connectivity across emerging markets and Asia. |
810 | |||
| 13 | 782 | ||||
| 14 | 752 | ||||
| 15 |
Zenzerozenzero.co.uk
Design tailored IT infrastructure solutions using proven technologies to optimize client operations and maximize value from resources. |
688 | |||
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 |
Options Technologyoptions-it.com
Provides global financial technology solutions specializing in banking and trading infrastructure, integrating high-performance networking, cloud, security, and AI technologies. |
641 | |||
| 17 |
Telef贸nica Tech (EN)telefonicatech.uk
Specialist provider of integrated technology solutions including cybersecurity, cloud, data, AI, business applications, and digital workplace services. |
557 | |||
| 18 |
Fortray Global Service Limitedfortray.com
Provides services for digital transformation, cybersecurity, cloud, and managed IT. Also develops talent through tech and AI bootcamps for various digital skills. |
547 | |||
| 19 |
Methodsmethods.co.uk
Digital transformation partner for UK public sector, specializing in helping government agencies improve services through innovative technology solutions and cost-saving strategies |
534 | |||
| 20 |
SecurityHQsecurityhq.com
It is a global cybersecurity company providing tailored solutions to enhance security posture and maximize cybersecurity investment for organizations. |
503 | |||
| 21 |
Resillionresillion.com
A global leader in quality engineering, testing, and cyber security services, providing end-to-end digital testing solutions with a dedication to success and cutting-edge technology |
479 | |||
| 22 | 454 | ||||
| 23 |
GL Noble Dentondnvgl.com
Global provider of risk management services dedicated to safeguarding life, property, and the environment, with a focus on safety, innovation, and sustainability |
423 | |||
| 24 |
CRESTcrest-approved.org
An international organization ensuring quality in the global cyber security industry through accreditation, certification, and collaboration with various stakeholders worldwide |
419 | |||
| 25 |
MBAmba-it.com
IT solutions and services provider in London, specializing in complex applications, enterprise capabilities, and exceptional service for businesses in finance, legal, and insurance sectors |
388 | |||
| 26 |
Police Digital Servicepds.police.uk
The company coordinates and delivers digital services for UK policing to use technology for public safety, working with various organizations |
379 | |||
| 27 |
S-RMs-rminform.com
It is a global intelligence and cyber security consultancy, providing critical decision-making support, resilience building, and crisis response for clients. |
373 | |||
| 28 | 
Pen AIgudens.com
AI-powered writing tool with features like idea generation, content expansion, research assistance, overcoming writer's block, style control, grammar checking, and plagiarism detection Join the writing community |
363 | |||
| 29 |
Security Blue Teamcentri.org
It provides practical defensive cybersecurity training, free courses, certifications, and platforms like Blue Team Labs Online and CySec Careers. |
354 | |||
| 30 |
Kochokocho.co.uk
Empowering ambitious companies with Microsoft cloud solutions, identity, mobility, connectivity, and cyber security services for secure growth and business success |
332 | |||
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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.
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