Correct interpretation of Rule 5 would actually allow you to fly as low as you liked - so long as you were not less than 500' from people,vehicles, vessels and structures....e.g. in a suitable military low level training enclave.
That'll be the yellow peril based on the Fournier RF6 motorglider then. The same aircraft as the US withdrew from military flying training service........
Surely someone in UK plc can design a decent affordable military elementary trainer which isn't hampered by glider roll rates? Perhaps a development of the Europa might fit the bill?