englishal
99.99% of the aircraft which cannot fit Transponders are not on a std CAA C of A. As FLARM can come as a very small portable unit it would not require any installation and would not be included in any empty weight calculation. The PFA/BGA/BMAA could make it happen, but some “assistance” from the CAA would help. I would fall outside the group as I have a Transponder, but if as you say the above group had the units, I would get one anyway.
Rod1