The law states "You MUST NOT carry a passenger unless your cycle has been built or adapted to carry one."
So if I strap a cushion onto my rear rack to give someone a backie, I've adapted my bike, right? In fact, a rear rack by itself is meant to carry loads, one of which could be a human being, so that in and of itself could be a pertinent adaptation.
Having given and received backies more than once in this country, I find this law a trifle silly.