Reason is, you need a WAAS/EGNOS compatible GPS to certify for full function.
Not correct.
The humble Garmin portables all have WAAS/EGNOS capability but are NOT certified and not allowed to be connected to an ADSB out transponder in the UK at this time, which what we are talking about here, the trials are to allow such non - certified units to be connected
The rate the CAA gets to grip with stuff (aka years after the rest of the world seems to just get on with it) Is frankly appalling, so I expect at least another year to go by before they actually schedule a meeting to decide when the meeting to decide a time scale for the next meeting, will happen