The real solution (for a problem which doesn't really exist - en-route mid-airs are very rare) would be mandatory Mode C (for all aircraft with an electrical system) and traffic data uplink like they are getting in the USA. That would start a nice thread here I am sure.
It already did! I'm not going to look for the thread, but it got very excited - all the Mode S and ADS-B etc stuff was very thoroughly aired about six months ago.
I saw, at the PFA Rally, a "low-cost" mode C transponder on the CAA stand. Nice idea, but only a two-year life before it would have to be thrown away and replaced by a Mode S.