There are very few GA mid air collisions. All would have been less likely if TCAS was fitted to one or both aircraft; Mode s (instead of mode c) would have made no difference. The Tornado / C152 incident would have been avoided if the Tornado had had TCAS.
The CAA proposal makes mode S compulsory, not radio or TCAS, and our most likely mid air is with other GA traffic.
The RAF are fitting TCAS to some of its fleet, but the time frame if many years and it is starting with the large transport types. Stay above 2000 ft AGL and your chances of meeting the RAF in the open FIR are insignificant.
Rod1