this question all ways comes up and i can tell you in Ireland it OK to fly a N reg on a UK PPL in Ireland so i am sure it would be OK to to the same in UK. the reason i am sure of this is on the last AAIU (Irish accident report) it stated that a pilot who was flying a N reg EC120 on a UK PPLH in Ireland was properly licenced. search the AAIU website and it is one of the last entries. its only after an accident these things come a problem