It is in your interest to ensure all works are overseen by the local building control department who will then give you a Completion Certificate – a competent builder will be aware of this and offer so to do.

Builders can join a self certification scheme https://www.gov.uk/guidance/competent-person-scheme-current-schemes-and-how-schemes-are-authorised#types-of-building-work

The government runs a trust mark scheme for builders https://www.trustmark.org.uk/

The Federation of Master Builders are worth looking at https://www.fmb.org.uk/

A builder should always have in place full public and employers liability insurance 
https://www.swinton.co.uk/business/trades/builders-insurance/?aff=ppc/google/swintoncommercial/trademaninsurance-bmm/buildersinsurance/buildersinsurance_broad&gclid=CJmf6eazj9ICFUg8GwodWR0CKw