FLARM was pushed very hard by the BGA in the 1st Mode S consultation. The CAA’s response to the consultation formally rejects it as an acceptable solution for a number of reasons.
I know some of you would love to a nice black box which will guarantee your collision avoidance with no pilot effort but it just is not going to happen. The CAA has stated that it will not mandate any form of transponder and most flying machines do not and probably never will have one. The CAA took its current position because it became convinced that it was not technically feasible to fit a transponder to most flying machines. No transponder = no warning = work on your look out. The same is also true of radios, most do not have one.
Next time you are in good VMC make a note every time you spot another flying machine and make a guess as to weather it has a transponder or not. I assume traditional group a has, LAA types 25%, and micros, gliders, hang gliders etc 0%. I think you will find that a very large % of the things out there have no transponder and are not likely ever to have.
Rod1