The logical thing would be ADS-B, implemented using the Mode S data channel.
This is what the rest of the universe is currently going for.
You start with a Mode S transponder (which nearly all of us will have to fit eventually ) and you connect it to your existing GPS (if you haven't got a GPS then forget any possibility of a low cost effective anti collision system). The transponder will then continuously radiate your GPS position. Anybody with a Mode S can do this right now -
At a presentation by the CAA they spoke about ADS-B in rather scathing terms. In particular they reminded the audience that as well as paying for the main kit, the GPS itself must be 'an approved' one. So those of us with low-end Garmins, FlyAngels or even the SkymapIII would have to reequip with an expensive (and thirsty) GPS