I'm not an NPPL holder but I remember this came up a while ago and somebody pointed out, then, that an NPPL holder has to remain within x miles off the UK shoreline, or something like that. If that is closer than the FIR boundary, the switchover has to happen earlier (or later, when coming back) but the idea stays the same.
Switching PIC in-flight is still a bit dodgy though. Suppose the first P1 flies the aircraft into a situation where the second P1 is not happy. Does that second P1 accept command of the aircraft then, or does he refuse to take command? And what happens in the latter case?
Oh, and some clubs (like mine) require a pilot who wants to command/fly from the RHS to get a specific RHS checkout. Might be due to insurance reasons or just plain common sense: instruments in a typical GA aircraft are located LHS so from the RHS they're harder to read, plus you get a parallax effect. And you need to control throttle with the other hand in most aircraft.