There still are a few issues with air cadets flying under BGA rules.
Solo and instructor standards differ greatly between the BGA and Air Cadets. Presumably if flying under BGA restrictions cadets would have to be trained to a considerably higher level to solo. The same applies for instructors, air cadet instructors aren't qualified to BGA standards and would require retraining. You could use BGA instructors but I don't think there would be enough of them.
Plus, operating air cadet gliding with BGA instructors will strain the resources of most sites. I'm not sure paying club members would be happy having their site taken over by the air cadet organisation on a regular basis. The strain on resources would seriously disrupt operations.