Isn't this one of those cases where 2 out of 3 will do.
By this I mean if you have 2 of the following 3 then its legal.
Uk Licenced
Uk Airspace
Uk Reg Machine
I can't remember where I got this from so it could be complete tosh, but I'd dig further before commiting to 'N reg' Training just for the sake of flying your N Reg Aircraft.
Remember also training on N reg types in uk airspace needs permission of the secretary of state for transport (memory again, but someone like that).
V.