You'll usually have access to insurance-backed warranties and a robust complaints procedure to use in the unlikely event that work is found to be non-compliant.

An installer registered with a competent person scheme will notify the local authority on your behalf and will issue you with a certificate on completion. This can be used as proof of compliance. It'll also show up on a solicitor's local authority search when you sell your home.

If you don't use a registered installer you'll have to submit a building notice or full plans application and pay a fee to building control for inspection of the work you've carried out.

How to find a registered competent person: - 

for electrical work, visit the competent persons register
for roofers, visit competent roofers
for all other types of work, contact the competent person scheme operators

For more information, visit GOV.UK.