Radarspod
The LAA won a concession to allow mode c transponders to be fitted to permit aircraft till at least 2012, I am not sure if that was extended beyond permit aircraft or not as I am no longer directly involved.
Rod1
The LAA govern Annex 2 aircraft, which operate under an entirely different system. Gliders (except for a few vintage ones) have to comply with full EASA airworthiness certification. Since the majority of gliders were entered onto the EASA system with a wide range of different instrument and electrical fits, a generic mod to cover a number of gliders of the same type is not possible. I have had a long discussion with TRIG on exactly this subject. The regulatory obtacles are vastly greater than GA fitting 'portable' FLARM.
As for Lithium batteries, their fire hazard record is well documented, and fitting one in an EASA certified glider would be seriously illegal.