Switching Software Vendors: Handover and Reversibility Audit Cost in London (2026)

The verdict in three sentences

Switching the vendor behind a business application in London should be run as a service continuity project, not a commercial break-up. A realistic 2026 budget has three lines: a code and infrastructure audit at 2,600 to 7,000 GBP plus VAT, a 4 to 8 week transition, and a new application support contract at 700 to 2,600 GBP a month. The hard part is rarely technical: it is recovering access to repositories, cloud accounts, DNS and payment accounts before you tell the outgoing agency anything.

What to recover before you serve notice

Most handovers that go wrong share one pattern: after the termination letter, the client discovers that the domain, the AWS account or the GitHub organisation belongs to the agency. A typical UK support agreement carries a 1 to 3 month notice period, and that is exactly when the outgoing supplier is least motivated. Build the inventory first, quietly, by requesting access "for the internal team".

One rule: every account that produces an invoice must be in your company's name with at least two internal administrators. If that is not true today, fix it before the audit.

The handover audit: scope and cost

The audit answers one question: can the application be taken over as is, does it need fixing, or does part of it need rewriting? It drives the new vendor's quote. Without it you will receive support proposals that are either padded out of caution or underpriced and renegotiated at the first incident.

A mid-sized business application (40 to 80 screens, a PostgreSQL database, an API) typically lands at 3,900 to 5,200 GBP for a full audit. Above 7,000 GBP, ask why: either the scope is broad (several apps, native mobile) or the auditor is pricing a disguised rewrite.

The 4 to 8 week transition

A clean transition has four stages. Weeks 1 and 2: recover access, stand up a staging environment with the new vendor, run a dry-run deployment. Weeks 3 and 4: knowledge transfer with the outgoing agency, ideally 2 to 4 sessions of 2 hours billed on time and materials (budget 900 to 2,200 GBP), and takeover of open tickets. Weeks 5 and 6: the new vendor owns fixes, the old one stays on standby. Weeks 7 and 8: full production cutover, rotation of every password and API key, revocation of the agency's access.

Your current contract's exit clause is your leverage. If it exists, it usually requires delivery of source code, documentation and transfer assistance. If it does not, UK copyright law works against you: without a written assignment, the contractor who wrote the code generally owns it. Have your solicitor check this before serving notice.

For software you license rather than own, a source code escrow agreement (NCC Group and similar providers) deposits the code with a third party, released if the supplier fails. Expect 1,000 to 3,000 GBP a year for a standard single-beneficiary agreement.

The new support contract: what to demand

Write into the new contract: client ownership of every account, assignment of IP in all code produced, an exit clause capping transfer assistance at 20 days at a fixed rate, and measurable service levels (response time, uptime).

Kolonell can audit an existing application and price a takeover; for an international client the scope and support level are set in a quote after the audit. See our custom web applications.

Mini case study

Oliver, managing director of a 45-person B2B services firm in London, pays 2,100 GBP a month to an agency that takes 6 days on average to fix a blocking bug. His booking application drives 1.0 million GBP of annual revenue.

Handover cost: audit 4,500 GBP, knowledge transfer with the outgoing agency 1,500 GBP, staging and cutover 3,900 GBP. One-off total: 9,900 GBP.

New contract: 1,650 GBP a month with a 4 hour response. Monthly saving 450 GBP, or 5,400 GBP a year. More importantly, two major incidents last year each cost about 3 days of lost activity: 3 x 1,000,000 / 250 = 12,000 GBP per incident, 24,000 GBP in total. If the new response time cuts that risk by two thirds, the combined annual gain exceeds 21,000 GBP. The handover pays for itself in under 6 months.

FAQ

How long does a vendor switch take without downtime?

Allow 4 to 8 weeks from the start of the audit to revoking the old agency's access. A simple app can move in 3 weeks; a system with a mobile app and payments is closer to 10 weeks.

My agency refuses to hand over the source code. What now?

First reread the IP and exit clauses in your contract. Without a written assignment, a solicitor's letter costs around 400 to 1,200 GBP and resolves most cases; litigation is the exception.

Will we need to rewrite the application after the handover?

Rarely. In about 70 % of audits the recommendation is to keep the existing code and pay down technical debt gradually, using 10 to 20 % of the support budget. A rewrite is justified when the framework is no longer maintained or security is structurally compromised.

Is code escrow worth it in the UK?

It costs roughly 1,000 to 3,000 GBP a year and makes sense mainly for licensed software that is critical to operations. For bespoke software whose IP you own and whose repository you control, it is usually unnecessary.

How do we avoid ending up here again in three years?

Keep every account in your name, require updated documentation every quarter and run a backup restore test every 6 months. Those three rules roughly halve the cost of any future transition.

Let's scope your project. We audit your existing application, price the takeover and plan a no-downtime cutover over 4 to 8 weeks. Detailed quote within 48 h. WhatsApp +221 77 596 93 33.