A surprising number of invoices go out with no payment term stated at all - which leaves "when should this be paid" entirely up to the client's judgement. Setting a clear term is one of the simplest changes that speeds up payment.
There is no automatic default
Unless a contract says otherwise, UK law treats an unstated payment term as 30 days for business-to-business transactions - but relying on that default is worse than just stating your own term clearly, since not every client knows or checks that rule.
Choosing a term that suits you
7 days: reasonable for smaller jobs, ongoing/repeat clients, or when your own cash flow is tight
14 days: a common middle ground that still reads as professional and reasonable to most clients
30 days: standard for larger commercial clients, especially ones with a formal accounts payable process that runs on a monthly cycle
There is nothing unprofessional about 7-day terms for a sole trader - many larger companies use short terms with their own suppliers specifically because it protects their cash flow, and you are entitled to the same logic.
Say it once, clearly, on the invoice itself
State the term as an actual due date ("Due by 14 March 2026"), not just a duration ("Due in 14 days") - a specific date removes any ambiguity about when the clock started, which is exactly the kind of detail that makes chasing a late payment straightforward if it comes to that.