At the moment on a PT C of A the engine can run untill the manufacturers TBO and then provided the terms of AN 35 are met the engine can run untill TBO + 20%.
It can run on a private C of A untill an engineer is no longer happy to let it.
For economic reasons I would not run an engine past TBO + 20% but would not object on technical grounds.
A overhaul + 12 year rule seems to have been set by the CAA/EASA for public transport but with little understanding of the implications for GA in the UK.