From the regulation:
(a) The privileges of the holder of a PPL(A) are to act without remuneration as PIC or co-pilot on
aeroplanes or TMGs engaged in non-commercial operations.
So if its remunerated or deemed a commercial operation you cannot do it on a PPL.
There are numerous hazards associated with parachute dropping, and glider towing, therefore to ensure that the pilots involved have a good knowledge and experience of the activity, then it is in their best interest to only use pilots who already have the requisite knowledge and experience. Jo Bloggs who just fancies some some flying with them without the experience is a potential liability and best avoided.