Northsouth
I can think of a few sources of primary-only contacts in your neck of the woods which SSR and TCAS won't help you with, and there's widespread doubt that, even if the UK officially sticks to mandatory Mode S implementation from March 2008, Paraglider Pete will (a) be in a position to shell out the money required and/or (b) give a toss about what the CAA says he should carry
When mode S comes on line, Mode 3A will be redundant. At present there is no way of knowing whether an ac in Glass G is IFR or VFR; however, if VFR ac were required to squawk Mode 3A 7000 when operating autoniminiuosly and 6000 when they were IFR and individual control positions were given 2 squawks 1 VFR the other IFR, all controllers would know the flight rules as well as the callsign of the ac on the screen. In this way we could get rid of RAS, RIS and FIS and concentrate on VFR and IFR and get ourselves back in line with the rest of the world. IFR ac would be provided instructions to separate them from other IFR ac and information on VFR ac with avoidance on request (as they are in class D) and VFR ac that wished to recieve a service would be given traffic information on other ac as far as practicable. In one go you have turned an unknown enviroment into a known enviroment as far as practically possible whilst maintaing the freedom that the open FIR affords. You would be able to separate ac that declare themselfs as unable or unwilling to follow the see and avoid principle and on the other, there be no requirement to muck about with light ac that are quite happy flying VFR. There will always be the problem of people not buying the kit but in such circumstances or when their kit is u/s you simply say that they are not allowed to fly IFR.