Can you just imagine the bureaucracy of trying to get self-built aircraft used by the ACO. They would take a relatively simple and proven system that is used already and make it so difficult that it wouldn't be worth it. Who would accept the risk? Who would sign it off? Who would be qualified to inspect it (not the LAA inspectors etc cause they're not as qualified as RAF Engineer Officers you know).
That would really be putting the fox in the chicken coop.