Assuming that the drive to transponders is being done to better manage the airprox situation (particularly the issue with CAS busts by non transponding GA) I don't think mandating Mode C/S above 10,000ft would help in the UK. Almost no GA traffic flies that high.
Even at 5000ft there is hardly anybody.
Adopting the US model, of Class E to 17999ft and Class A from 18000ft up, that would be something else, but nobody in the UK is going to be that bold. Even the French model, Class D above FL115, would be too much to swallow.
One would have to completely re-do the whole airspace thinking to do this.
Incidentally, referring to that paraglider picture above, is there a proposal to mandate Mode S for these?
