Any work carried out on an aircraft in the UK that is to be used for the purposes of 'Public Transport' (eg private hire) or 'Aerial Work' (eg paid instruction and aircraft hire) requires a Certificate of Release to Service (CRS).
The CRS can only be signed by a licensed engineer, so as has been said already, anyone can do the actual work but the aircraft can't be used for those purposes unless the work has been signed for by a licensed engineer.