clunk makes a valid point given the overall global marketplace that we operate in.
The only way to limit supply is to focus on the ability of candidates, e.g. currently, the real limiting factor is the ability to fund £80-£100k.
If those candidates had to have tighter rules, e.g. 4 x A's at A-level, then supply would fall off a cliff. The issue is that airlines seem broadly happy for mediocre cadets, especially given the training cost & risk is with the cadet.