My understanding is that you can legally relinquish control and command to another suitably qualified (this means in currency including 3to/landings) pilot
I agree totally. There is some precedent for this too, in cases where e.g. an instructor flies as a passenger for a bit, and the training part of the flight starts outside the UK airspace boundary.
however it is likely that you will have not fulfilled your obligations if you are in a club AC, as the stated AC commander will probably be already signed as such in the tech log
Well, yes but that is totally irrelevant to legality of the flight. That is just [non]compliance with some club rule, and might impact an insurance claim. I would assume that if there was a prang, the PIC in the club records would accept responsibility even if somebody else was actually on the controls. But one cannot guarantee human integrity in the face of adversity or poverty
