We used to have three types of C of A in the UK. Private, Aerial work and Public Transport.
Public transport aircraft are required to be in a controlled environment and all the inspections/work completed by a Part 145 organisation. This would only be required if used on an AOC etc.
This is not the case for Aerial work (rental or flight instruction in this case) they must however be commercially maintained, the last inspection must have been completed by an approved organisation, Part F, Part 145 or a licensed aircraft engineer.
The engine must be within TBO or +20% if GR24 is applied. All the smaller bits also need to be within hours or calendar, the extent of this seems to vary a little and is decided between operator, engineer and CAMO/Part G doing the airworthiness review.
If you have had tbo + 20% out of an engine you have had your moneys worth, who said flying would be cheap
