I think this 'one A-level is harder than another A-level' is BS. ST69 should play to his strengths to gain the best grades he can. In this case, Media Studies and Psychology counter-balance Maths and Physics and demonstrates a rounded educational base which will impress OASC. Switching from integrating equations to lengthy, detailed essay-based analysis is plenty mental gymansticism.
As has been said, however, academics are a single part of the package. Make sure you're getting involved with all the adventurous, sporting and community stuff you can also, because if you don't then someone else will be. It's a competitive game. Try and fit some computer games in as well for the aptitude tests- flying ILSs on Flight Sim is good for this, and assists with your aviation knowledge.