Many of the recruitment folks I've spoken to have stated their ability to recruit for certain branches isn't a problem (e.g. Pilot). So for some branches, the Services can afford to be extra stringent on entry standards.
The real issue is the training system getting enough qualified branch personnel out the other end to do the role - that's often where things are being held up.
Some branches are struggling to get them in the recruiting office door though, so attracting the right people is still a problem.