from what ive picked up, you dont need to convert your ppl to uk. so long as the aircraft is g reg and flown in the uk, you should be fine. the info i have been given is from a south african fto, but there shouldnt be any reeason why the same doesnt apply:
The South African (ICAO) Private Pilot Licence is valid for entrance to the UK CAA exams and for flights in UK registered aircraft within United Kingdom airspace including the flight training and tests; so there is no need to convert to a JAR-FCL PPL (A) before applying for the JAR CPL/ IR.
hope this helps and doesnt confuse things!