Have a read of the part P document.
Electrical safety: Approved Document P - GOV.UK - https://www.gov.uk/government/publications/electrical-safety-approved-document-p
It is a legal requirement that work carried out in dwellings complies with Bs 7671, and that certain works in dwellings are notified to building control. The only cost effective way of notifying is through one of the schemes.
Most work will require an EIC or MW cert issued to the customer, and you will then notify building control (if required) through your scheme.
The term 'signing off' is often used but is rather misleading.