The cost benefit is interesting. Assuming about 30k aircraft in the UK needing to install kit (LAAs, Micros, Gliders, and some Spam cans) and a rough £2500 cost (assuming a new unit, certified, and allowing for some aircraft to have considerable cost beyond basic) and say £100/year in inspections and maintenance costs ...
That gives an investment of £75 million pounds plus a running cost of £3m pa. And for that, the investor has to rely on the person he is going to have a collision with having invested further money in some additional technology, or being able to get a radar service (the existence of which doesn't appear to have prevented the recent Coventry collision) - So by no means a 100% reduction in risk. Based on past history of midairs they rarely involve aicraft fitted with TCAS or even Skywatch). At the apocryphal £1m/life, this doesn't seem to be a great investment proposal.
On the other hand, if everyone did have a transponder, then a lot of people would spring for some kind of proximity detector. And even better, if these transponders were allowed to radiate a GPS position (without being a major mod with the administrative cost associated with that!), then we would have a very slick system.