Good common law instincts from FFF: the general rule still holds true that, in the UK, nothing is illegal unless there's a law saying so. You will comb the ANO and related materials in vain for anything which says that you can't let granny wiggle the stick about. Anyone daft enough to let granny try to land the thing has only themselves to blame if things go horribly wrong with insurance etc.
Slightly off thread, but I was depressed by IO540's suggestion that breaches of the rules about costs sharing are commonplace. If this is true and there really are lots of PPLs mean and stupid enough to charge people for flying, I will sign up my granny (unlicensed but enthusiastic wannabe kamikaze pilot) to be their next passenger: "here sonny, I'll take over while you count all those fivers I just gave you"