These TCAS debates run for ever but I think a fair summary is that the driving force behind it is to get something that will trigger the existing TCAS systems on commercial jets.
That narrows the technical options to current Mode C or current Mode S. The CAA appears to think that Mode S is necessary because Mode C would generate too many returns and swamp the radars (that's my understanding).
There are other ways to get traffic warnings but they don't meet the above requirement so they are a bit academic.
Also nobody in authority cares about GA-GA conflicts and I think that's probably reasonable. They are extremely rare and technical solutions based on existing transponder technology are expensive, £10k as mentioned above. They also don't do much because so many planes are not transponding.
It's possible that if/when Mode S is made mandatory for VFR, other solutions will emerge on the back of that (Mode S provides a means of unsolicited broadcasting of the aircraft position) but I don't hold out much hope for a universal adoption because most of the GA scene, UK and elsewhere, won't spend any money they can avoid spending.