According to the blurb in the front of my CAA logbook, you certainly can log P1 (shared) time but you can only log the time you are actually operating the aeroplane.
I agree that this whole thing is a typical example of the Authority not giving clear guidance; some is obvious, some is open to interpretation, some is totally unclear. Personally I'd be happy for (SEP):
PIC - you are the Commander and anyone else is a passenger.
PU/T - you are being taught a new skill by an FI.
P1U/S - you are doing something under the supervision of someone else who is the Commander.
PS - Aren't PFA coaches now going to become CRIs? In which case they may not be paid unless they hold commercial licenses?