For what little its worth, my reading of CAR 5.81 says that the flight review has to be conducted by an "approved person" and in a particular type of aircraft (5.81 (2)) but makes no mention of the registration of the aircraft or location of the flight. So if you can find an instructor who has the suitable approval from CASA and the legal right to instruct in the UK it seems plausible.
Mind you if I was doing this and had lined up a suitably qualified instructor and G-reg aircraft I would probably still want to check with CASA.