How can a transponder possibly be an issue in an Air Cadet glider? For club use, battery capacity on an all day cross-country could be a problem, but for a glider doing circuits its no big deal to swap batteries every few flights. A quick check suggests a mode S glider transponder might use 0.28A when active, so a typical 7AH (Amp Hour) gel cell battery should be more than adequate for a couple of hours reliable operation. As few cadets are likely to be at the top end of the weight range, carrying extra batteries instead of ballast weights would be a possibility too.
With regard to sites, the technology already exists to use the width limited ones; namely a retrieve winch. A 90 to 120 second launch cycle is easily achievable with one of these, and the fact that the parachute end of the cable is pulled back onto the launch run after release minimises the likelihood of the cable going where it shouldn't. An added benefit for site owners that are precious about the condition of their airfield, is that the cable only needs to be pulled out by a vehicle once a day, so the cable run doesn't get chewed up. Sure, its an extra piece of equipment to learn to operate, but one only has to go to Long Mynd to see what a slick launch cycle it offers.
Given how gliding started, its quite bizarre that Wales is considered unsuitable as it is too hilly. If the Welsh Air Cadets do lose their sites and still want to fly, perhaps they should speak to the Joint Services Hang Gliding and Paragliding Centre (JSHPC) near Crickhowell. Obviously hang glider control input is different, but so are bicycles and cars. Flying a hang glider on tethers is very safe and most definitely a group activity which will ensure the passengers are asleep in the minibus on the way home.