Supposed I'm being checked out on an aircraft by another PPL (the owner, say). If I'm not P1, then the owner is, and that means that he has to pay at least an equal share of the costs. I don't think there are that many owner-pilots willing to pay for checking out other pilots...
Not, IMO, under UK legislation. If it's a shared aircraft operated on a private C of A, any of the co-owners can give the valuable consideration to the group funds. If it's hired to you by the owner on a PT C of A, provided the owner does not charge for his services as pilot (and proving that may require care) you are still paying "for the primary purpose of conferring on a particular person [you] the right to fly the aircraft on that flight". The person who pays does not have to be the commander.