Engines - it was a generic comment on ‘witch hunting’ and not aimed at anyone in particular :-)
However, I’m afraid you are quite wrong here with this
You ask why were the aircraft ever put on the military aircraft register? The short answer would be because they were bought using money that was allocated by the Government for military aircraft. They were bought by the MoD to be operated by an arm of the RAF. If the RAF operates aircraft, they have to be on the military register.
Our current Grob Tutors are on the civilian register and we have operated some Front Line types on the civvy register in the past as well. Further I believe that Shadow and Voyager are operated to Part-145? The ANO allows for civil registered aircraft to be flown by military aircrew when on military duties. However, to make it simple, for Air Cadet purposes, I would propose that any civvy reg gliders/TMGs be flown by staff with appropriate civvy licences - again our Military Regulatory Publications (MRPs) make provision for this if required.
So it could be done VERY easily IF there was an appetite to do so. But for some unknown reason it isn’t! Part of me suspects it is because corporate knowledge of civvy flying is so very low in military circles...