Should a UK practice buy AI software or hire an agency?

The most widely adopted artificial intelligence in UK businesses of ten or more people is a large language model, used by 18 per cent of them in June 2026, with visual content creation next at 16 per cent. Both are general-purpose tools. Neither knows anything about your practice or the standard your work is held to.

That is the honest starting point for a question partner groups now put to each other every quarter. A supplier quotes a monthly retainer to run a piece of marketing or client admin. A vendor quotes a per-seat licence for a tool that appears to do a version of the same thing for a tenth of the money. The pages published on this choice compare the two on speed and control, and every one of them is written by a party selling one of the options.

Two things settle it, and both can be checked this afternoon without asking either supplier a question. One is a division. The other is a clause.

The fork is accountability, not capability

Both options can produce the work. Only one of them owes you anything when the work is wrong. That is the distinction the published comparisons skip, because it is not a feature either side can advertise. Four questions route the decision, and they are worth answering in order rather than at once.

The cost crossover is a headcount, not a date

Per-seat software and a fixed retainer cross at one number, and that number is the headcount that would need a seat. Divide the monthly retainer by the per-seat monthly price and the answer falls out. A £750 retainer against a £20 seat crosses at 37.5 people. Below that the licence is cheaper and above it the retainer is. No amount of discussion moves the point at which they meet.

The figures below are illustrative. They hold the retainer at £750 a month and the seat at £20, and vary only the number of people who would need one. Substitute your own two numbers and only the crossover moves.

The two contracts do not promise the same thing

A services contract in the UK carries a statutory standard of care whether or not anybody negotiated for it. Section 13 of the Supply of Goods and Services Act 1982 provides that where a supplier acts in the course of a business, there is an implied term that the supplier will carry out the service with reasonable care and skill. It governs business to business work, which is why a retainer that quietly underdelivers is a contractual matter rather than a disappointment.

A software licence is drafted to reach the opposite result, and it says so on its face. Anthropic's published commercial terms state at D.3 that it is the customer's responsibility to evaluate whether outputs are appropriate for their use case, including where human review is appropriate, and that factual assertions in outputs should not be relied on without independently checking their accuracy. The warranty section disclaims implied warranties including any arising from statute. That is the ordinary shape of the industry rather than a criticism of one vendor, which is why the useful move is to open your own supplier's equivalent clause before signing.

One document buys capability the practice must supervise, the other an outcome somebody is answerable for. It is why a retainer has to name units rather than service categories and why two quotes for one brief can be eight times apart before either is divided into days.

What you are holding when either one stops

A licence that lapses takes the configuration with it. The seats switch off, and so do the saved prompts and whatever tuning the practice accumulated over two years, unless somebody exported it while the account was live. An engagement that ends should leave the firm holding the work product, but only where the contract assigns it. That clause deserves more attention than the notice period, which only decides when the invoices stop.

When you should buy neither yet

Two situations make the whole comparison unanswerable, and neither of them is a supplier's fault.

• Nobody has written the task down, so the firm ends up paying one supplier or the other to discover what the job was, which is the most expensive way to find out
• Nobody inside the firm has an hour a week to read the output. A tool nobody checks produces work nobody trusts, and an unattended retainer converts into whatever the supplier finds easiest to deliver by about month four

A page that talks a partner group out of hiring anyone costs us the fee, since we are paid by the agency and only when a relationship works. It is also the honest answer often enough to be worth writing down. What we assess before recommending anybody, including the cases where we decline to introduce anyone, is set out for accountancy practices, recruitment agencies, tax advisory and financial advice firms, and the arithmetic for the wider version of this question is in break-even on one additional client.