I think the NATS guys have made a bit of a pigs ear of organising this. No wonder people don't know what's going on. I had a similar issue but kept pushing it.. they need volunteers!
There were loads of delays , as there usually are. As they worked through it and they got clearance with the CAA... then got the LAA/BMAA to sort out the minor mod process, it was pretty much June by then. I only got mine finally approved at the end of June.
Since then, they have told me they're extending it until August 2016. So if you still want to do it, I'd suggest you email
gps.trial@nats.co.uk and poke them.
If you have a Garmin style GPS these have the RS232 output which interfaces directly to your transponder... maybe time to dust it off :-) I'm using a GPS 96C.
The way I see it is that if everyone who has mode S is also using ADS-B out (+pilotaware box), while the folks who cannot install a transponder have a pilotaware, then between us we're covered - everyone can track everyone else.
NATS have openly said that they do not use ADSB out for traffic management (at least at GA/sport flying level).. I'm not sure what their longer term view/goal is.. I suspect they don't want to rely on uncertified GPS data for traffic management, so they'd ignore us anyway... this is more for helping out GA/sport flying.