My understanding is that parachutists were the one group that the CAA were going to exempt from Mode S (after a fair bit of discussion).
There was also a proposal that aircraft that stayed in a circuit and never 'conflicted' with other aerial vehicles would also not need to have Mode S.
But, the CAA originally were of the opinion that all aerial vehicles would have to be Mode S equipped - gliders, paramotors, hang-gliders, microlights, gyroplanes, sport & recreational aircraft etc etc