I used to fly gliders in Switzerland and most gliders there have an anti-collision system invented by a swiss-boffin. Like FJs, gliders are often flying in close proximity in thermals and so TCAS is useless. I can't remember how the little magic box worked - but it was very effective. It only cost a couple of hundred Francs - I'm sure the MOD could knock up something similar for a few million quid a piece.