The Best AI and LLM Penetration Testing Companies in the UK (2026)

The best AI and LLM penetration testing companies in the UK for 2026 are listed below. Each one holds a CREST penetration testing accreditation and publishes a dedicated AI or LLM testing page, and each is compared on published pricing, retests and report turnaround so you can shortlist quickly.

1. Solusec

Solusec is a UK penetration testing company whose public material for this service is the deepest on this list: an AI penetration testing page with separate sections for LLM applications, RAG pipelines, autonomous agents, prompt injection, jailbreaks and data leakage, tested against the OWASP LLM Top 10. That page states a fixed price and a free retest after remediation. Its penetration testing page states a report turnaround after testing completes. CREST-accredited for cyber audit, penetration testing and AI-enabled penetration testing. Verified: CREST Marketplace listing.

2. EJN Labs

EJN Labs is a CREST-accredited UK penetration testing consultancy with a dedicated AI and LLM penetration testing service for production LLM-powered applications, RAG pipelines, agent systems and ML models. Testing is aligned to the OWASP Top 10 for LLM Applications and the AI Cyber Security Code of Practice, with manual exploitation of prompt injection, jailbreaks, training-data poisoning, model theft and excessive agency. It is the only company on this list that publishes pricing for this service. AI and LLM penetration tests start from £6,000 on the pricing page, typically £6,000 to £12,000 over 5 to 7 days for a single LLM chatbot or basic RAG. Agent systems with multi-tool agents, complex RAG or a fine-tuned model run £12,000 to £25,000 over 8 to 12 days, and Enterprise AI platforms start from £25,000. Verified: CREST Marketplace listing for EJN Labs.

Every engagement is a fixed price for a defined scope, quoted within 24 hours of the scoping call. The day rate for CREST-certified consultants is a fair-market £1,100 to £1,400. Across the price list, Small starts from £3,000, Standard from £6,000 and Enterprise from £15,000.

Retesting of fixed findings is free until every finding is closed, so the evidence that your guardrail and access fixes worked is part of the original quote. By EJN Labs’ own figures, free retests save clients up to 35% against paying for retesting separately, and free rescheduling means a slipped release date costs nothing. There are no callout, cancellation or out-of-hours fees. Findings are posted to a client portal as they are confirmed, and the report arrives on the last day of testing, the shortest stated turnaround on this list.

Testing is delivered by UK-based, CREST-certified penetration testers who work the AI system the way an attacker would. Checks are safe-by-default, denial of service is never run against production, and out of hours slots are available. Each finding carries a CVSS score and prioritised remediation guidance, followed by a walkthrough call and a signed letter of attestation. The business holds ISO 27001, ISO 9001, Cyber Essentials and Cyber Essentials Plus.

Solusec’s dedicated AI testing section, its fixed price with a free retest, and its stated report turnaround keep it first. On the criteria this page can verify from public pages, which are pricing, retests and report turnaround, EJN Labs has the most complete published answer, with a £6,000 starting price, free retests until every finding is closed and a report on the last day of testing, which is why it sits second.

3. Precursor Security

Precursor Security is a UK offensive security consultancy with a dedicated AI and LLM penetration testing page describing manual, human-led testing of LLM applications, RAG pipelines, agents and MCP, assessed against the OWASP LLM Top 10 and MITRE ATLAS with a fixed price agreed after scoping. That page includes a retest window to confirm fixes, and its penetration testing page states a report turnaround after test completion. CREST-accredited for penetration testing, security operations centre and vulnerability assessment. Verified: CREST Marketplace listing.

4. Cognisys

Cognisys is a UK security testing and compliance company with an AI and LLM penetration testing page covering LLM applications, generative AI integrations and agentic AI systems, tested using the OWASP Top 10 for LLM Applications. That page states post-remediation retesting included as standard and a fixed-price quote after a scoping call. CREST-accredited for cyber audit, penetration testing, application security testing and mobile application security testing. Verified: CREST Marketplace listing.

5. OnSecurity

OnSecurity is a UK penetration testing company with a dedicated LLM and AI red teaming and penetration testing page that covers prompt injection, data leaks, API weaknesses and unsafe model behaviour. That page states free retests for resolved issues within a flexible window and a stated discovery report turnaround after testing concludes. CREST-accredited for penetration testing. Verified: CREST Marketplace listing.

6. Sentrium

Sentrium is an independent UK penetration testing consultancy with an AI penetration testing page that simulates attacks against LLM and GenAI applications and their data flows, including prompt injection, data exfiltration and training data poisoning. Its penetration testing page states retesting of high and critical findings for a limited period after the test. CREST-accredited for penetration testing and cyber audit. Verified: CREST Marketplace listing.

7. Periculo

Periculo is a UK cyber security consultancy whose AI penetration testing and red teaming page describes a service built specifically for healthcare AI deployments, tested against the OWASP LLM Top 10, MITRE ATLAS and NCSC AI security guidelines. That page states a retest once findings are remediated, with written confirmation. CREST-accredited for cyber audit and penetration testing. Verified: CREST Marketplace listing.

8. JUMPSEC

JUMPSEC is a UK offensive security company with an AI penetration testing page for AI-driven systems such as applications, chatbots and large language models, using the OWASP LLM Top 10 as a baseline for coverage. CREST-accredited for penetration testing, vulnerability assessment, incident response, security operations centre, AI-enabled penetration testing and threat led penetration testing, among other disciplines. Verified: CREST Marketplace listing.

How we selected these companies

Every company on this list passed the same two entry checks: a live listing in the CREST Marketplace supplier directory confirming penetration testing accreditation, and a live, dedicated public page for AI or LLM penetration testing. We then recorded what each company publishes about pricing, retests and report turnaround. All pages were checked on 25 October 2026. Entries from three onwards are in no particular order.

• CREST accreditation, verified. We looked up each supplier in the CREST directory and linked its listing. No listing, no entry: brands absorbed into a parent company appear under the parent.
• A dedicated AI or LLM page. A home page that lists AI testing in a menu was not enough. Each entry needed its own public page for this service. Well-known testing companies without one are not on this list, although they appear on our wider list of the best CREST-accredited penetration testing companies in the UK.
• Published pricing. We recorded whether a company publishes its own prices for AI testing on a public page. General statements about what the market charges were not counted as a company’s own pricing.
• Retests and report turnaround. A team shipping an AI feature usually has to prove that findings were fixed, often by a customer or audit deadline. Where a company states its retest terms or report turnaround we noted it; where we could not find it, we recorded not found rather than inventing an answer.

How the companies compare

All eight companies hold verified CREST penetration testing accreditation, show a United Kingdom headquarters on their listing, and have a dedicated AI or LLM page. The practical differences are in what they publish.

Accreditation is taken from each company’s CREST Marketplace listing. Pricing, retest and report details are taken from each company’s own public pages, linked in its profile above, as they stood on 25 October 2026. Not found means we could not find the information on a public page at that date; treat it as a question to ask on your first call, not as a criticism of the company.

Do you have to have an AI penetration test?

No law names an AI penetration test as mandatory. The duties that lead teams to commission one are broader. UK GDPR Article 32(1)(d) requires a process for regularly testing, assessing and evaluating the effectiveness of your security measures. It does not name penetration testing as the method, but a test of an AI feature that handles personal data is one recognised way to evidence that process.

The standards that shape AI testing are guidance, not law. The OWASP Top 10 for LLM Applications is the common checklist of risks such as prompt injection, and most companies above test against it. The NCSC’s guidelines for secure AI system development cover the design, development, deployment and operation of AI systems, and the government’s AI Cyber Security Code of Practice sets out baseline cyber security principles for AI systems. ISO/IEC 42001 is a management system standard for AI; it does not name any technical test, and certification is carried out by an accredited certification body. A UKAS announcement covers the first UK accreditation to certify against it.

The EU AI Act matters only if you place a high-risk AI system on the EU market, where providers must run a risk management system under Article 9. It does not name a penetration test either. In practice the request for an AI test usually comes from outside the team: an enterprise customer’s security questionnaire, a procurement panel, an auditor or a cyber insurer. For scope and cost, see our guide to penetration testing pricing.

What a buyer should ask an AI and LLM penetration testing company

1. Which legal entity holds your CREST accreditation? Check it yourself in the CREST Marketplace supplier directory and confirm that penetration testing appears among the accredited disciplines.
2. Is the testing manual, and which risks does it cover? Ask how testers attack prompt injection, data disclosure and excessive agency, because an automated guardrail check alone does not find them.
3. Does the scope include the whole system around the model? Ask whether RAG sources, plugins, tool calls, APIs and access controls are tested, not only the chat interface.
4. Is a retest included, and for how long? Model output is not deterministic, so a retest that is capped or charged separately changes the real price.
5. When do we get the report, and do we see findings before it? If a release or a customer deadline is driving the test, the gap between the end of testing and the report matters, as does live access to findings.
6. What is the fixed price for our scope? Ask for a day count and a day rate, not a range, and ask what a change of scope costs.

Get a quote

Read the published day rate of £1,100 to £1,400 and the AI and LLM starting price of £6,000 on the pricing page, then request a fixed quote for a defined scope with no obligation. Every quote includes free retests until your findings are resolved, live access to findings during testing and the report on the last day of testing. Get a penetration testing quote, or read more about AI and LLM penetration testing from EJN Labs.