It relies to a large extent on trust. You have to trust that the VFR (as it will almost exclusively be VFR) traffic knocking around under, say, the base of the LTMA, knows where it is and won't go where it shouldn't. I'd say you're on thin ice with the GA fraternity by suggesting compulsory ModeA&C, which would effectively close off swathes of Class G airspace to microlights, vintage traffic, gliders etc.
Don't forget, too, that if a VFR trundles into CAS by mistake, there's still the fact that they are operating VMC and clear of cloud and maintaining a proper lookout (!) which may allow some avoiding action if required.