A mini AOC would be a great idea but knowing human nature would result in massive protests to the CAA from holders of "proper AOCs".
It would be a great idea for another reason: in most places in Europe you can get tax free avgas (half price approx) on production of an AOC.
Whether you can in the UK is not widely known. I believe you can but not at the pump - you have to reclaim it separately. It would certainly explain why some schools have an AOC for charter which cost them a fortune but they seem to rarely operate it.
As for sticking a finger up to the CAA, the way the law works is that it states what is prohibited, and everything else is permitted. Someone might take a moral ground on it but that's a different topic. Everything not prohibited is 100% legal.