Pace, sorry to be pedantic, but you rolled two different things into one (perhaps because they are one, for rule-observing power pilots above 3000 feet).
1. Few gliders fly in cloud. When we do, we are usually out of the way of other things in cloud, owing to the altitudes/levels and locations concerned. Small number of events times small risk of encountering anything other than another glider = extremely low risk of encounters. Even so, it would not surprise me if one day the authorities mandate transponders for it. I would have had one by now if EASA etc. had not made it too difficult/expensive. I don’t know many glider pilots who take a similar view, however – but then I don’t know many who fly in cloud at all, these days. It was more prevalent when wooden, draggy, gliders were the norm, as Cats wrote earlier.
2. Flying in IMC, i.e. closer to cloud than 1000 ft vertically and 1.5km horizontally, we do all the time. It is fundamental to our means of locomotion. There will be an almighty battle if any one tries to stop that or mandate transponders for it. I understand EASA may have drafted something to that effect and negotiations are under way, but I am not in the loop.
For what its worth (not much – “history is bunk!”), the present exemptions predate my 39 years in gliding, but I gather that when the CAA or whoever after the war started to draw up legislation, cloud flying was then so fundamental to our means of locomotion, and caused no perceptible problems to other air users, that UK law became what it is and has stayed. Broadly speaking, we like to go up at or close to cloudbase, light GA tends to be at 1000-1500-2000 feet, hence usually below us when we go cross-country, and the heavier things, particularly CAT, tend to be higher. When its 8/8 and cloudbase is 1000 feet, we do circuits at our sites and don’t go cross country.
Chris N.