The UK regulations are quite clear regarding to the use of drone flying for commercial purposes. The Civil Aviation Authority (CAA) approves drone pilots by issuing a PfCO, or Permission for Commercial Operations. The PfCO is a legal requirement. It is gained via a training programme; flight checks ensuring safety and competency; the completion and CAA approval of an Operating Manual; and proof of relevant insurance.
The Operating Manual must contain a copy of the PfCO and insurance details and be made available to you, the client. If these are not available then alarm bells should ring. An up-to-date list of qualified and CAA-approved drone pilots can be found here.