Anyone can call themselves a roofer, so the checks are on you before you hire. The good news is that the checks are quick and they filter out the vast majority of poor operators. A contractor who passes all of them is very unlikely to let you down.
Accreditation
Check for NFRC (National Federation of Roofing Contractors) or TrustMark membership. Both require members to carry adequate insurance, operate legally and follow a code of practice. NFRC membership is verifiable at nfrc.co.uk — do not just take a logo on a van at face value.
Company and insurance
Confirm the business is registered at Companies House with an active status, and that the public liability insurance certificate names the specific company you are hiring. Ask how long they have traded — a 20-year local history is a strong signal a guarantee will still be honoured years from now.
The quote
Insist on a written, itemised quote specifying materials, scaffold, waste disposal, VAT and guarantee. A vague ground-level estimate texted over within minutes is not a proper quote. A contractor who surveys thoroughly and quotes clearly is one who will work to the same standard.