I don't recon that the CAA could justify insisting on all GA aircraft having a transponder, unless gliders were included. A glider could cause as much damage in a midair as a light GA aircraft, gliders are also allowed in cloud (UK only), where 'see and avoid' is impossible.
Gliders especially don't have the room for a normal transponder, and the aditional battery that would be required. The present £1000 cost is also prohibitive.
If a company like Garmin were to design a low cost, low power useage, Sqwak VFR ONLY (Or new designated code) transponder, it would become a possibility for the CAA to insist all aircraft were fitted with a transponder. However this device doesn't yet exist.
ickle