IO540 is quite right about the Europe wide bit, 8000 French micros are already except and the home built sector are pushing hard for the same treatment. Agreements allow all the French aircraft into UK airspace for a certain number of days (60?) a year.
If you force non radio Aircraft to fit mode S how are you going to know if it is accurate in the air? My only airprox in many years and 800 hours was in IMC. I was tracking via a VOR on a RIS in the open FIR. I was warned of unknown traffic indicating 2000 feet. I was 1000 feet above so carried on. I was in and out of cloud but above a solid layer, I came out of one bank just in time to see an aircraft flash under me left to right very close indeed. RIS indicated it was the unknown traffic so his encoder must have been out by at least 800ft. In this case both aircraft had mode C, but only one was talking to the local LARS. My examiner (it was an IMC renewal) did not report the incident. Mode S would not have helped in this case, which is also the case in all the fatal mid air collisions that have happened in recent years.
Rod1