porco,
I guess we would need to define 'a lot'. As for Mode C, I am sure it helps keep Class B surface areas down to reasonable sizes in the States, but remember most of the traffic skirting around or flying over Class B (no Class A until FL180 remember) is unverified mode C, so would it really help in the UK?
But you bring up a good point about mandatory use of transponders being used to keep surface areas smaller. It should be possible to drastically reduce all this CAS in the UK when they force mode S down everybodies throats. That might be an acceptable quid pro quo for those in the South East. Can you see this happening?