Doing Maths rather than (say) Biology or Chemistry would be a good idea. If you don't go into aviation it'll open up a few doors. Think about what you would like to study at university (yes, I know it's a long way off), because if you're into your sciences Maths will be a real help if you go on to do a scientific degree. Remeber that most cadets have a degree - besides that, uni is a laugh, and a degree will provide you with a valuable safety net.
Having done the Standard Three (Maths, Physics, Chemistry) I did Engineering at uni - but with them I could have done (among others) IT, Maths, Physics, Chemistry, Medicine (believe it or not, you don't need Biology to get on to a medicine course, just Chemistry). I wish I'd done a more arty subject as a fourth A-level, so I think that English would be a good idea, as would a foreign language, if you're that way inclined.
But ultimately Ian's right, do what you enjoy. That way you'll do better at them, and UCAS points mean prizes.
