OK, we have flarm, transponders, Pilot Aware and ADSB.
As I understand it:
A Pilot Aware linked to a Mode S transponder will give you ADSB-out. Not the super-fancy ultra-compliant one, but still an ADSB out. The connecting lead is about 100 quid. The Pilot Aware about 200. Bits and bobs another 100 and then the cost of the tablet. Full explanation here:
http://www.pilotaware.com/pilotawareintroduction/
So it brings together most of what TOO would like.
(of course, there will still be people with no transponder etc...)
We are putting a Pilot Aware ground station and a Flarm receiver at Strathaven Airfield, and quite a few of the folk here have been buying Pilot Aware.
I, personally, think it goes quite a long way to achieving a "common" system (yes, I am fully aware that it is actually a cobbled-together amalgam of all sorts of systems)