Well, I think we should have a rethink about all the factors involved - and also take the opportunity to harmonise/simplify things generally. We'll have to give way to some extent, but Mode S transponders on paramotors would be just plan madness.
What about:
Class A to C airspace - Mode S compulsory.
Class D - Mode S compulsory under IFR if > 2000kg. Mode C compulsory under IFR and < 2000 kg, Mode A compulsory if under VFR and < 2000 kg.
Class E to G - Mode C compulsory under IFR. No transponder required under VFR.
And how about a 6000ft UK-wide TA whilst we're at it?
Tin hat on.....