Hey davidson2006,
I am going to assume you have zero knowledge on the routes into aviation, and try to give you a realistic look at your prospects from your stage.
Your A level choices should be purely based on the subjects you enjoy. Simple as that. If you prefer French to Maths, then choose French. Don't choose a subject that you think will land you on a flight deck in 5 years time, as it will not be in your best interests if you don't really like the subject, and you'll spend a miserable two years studying a subject which you may well do badly in. The best bit of advice to be given here is, do what you enjoy. It's the best way to succeed at A level. Work hard at school and get the best results you can; sounds like a no brainer, but when you come to job applications, in any field (flying or not), this is when these sort of things become important.
The things you're doing at school sound good as well. The ATC is a good chance to get involved with flying (military flying is the best in the world, no doubt about it). Do the gliding scholarship, and get as much flying as possible in the Tutor as you can. See at the end of it whether you are still as enthusiastic about it as you were before you ever stepped inside an aircraft.
One thing I would say is have you considered University? Many people leave school at A level or GCSE level and expect to walk straight through an ATPL course and on to British Airways. This is a naive expectation. University will do a lot to broaden your experiences, perhaps give you an idea about other possible career paths, as well as general life experience and maturity. Remember, airlines will be looking for someone with a sensible head on their shoulders, not just a 'fresh out of secondary school' bright eyed, if somewhat immature young lad. University also gives you a chance to develop important social and other skills, such as teamwork, leadership, and communication. All in all, good life experience. Also bear in mind the University Air Squadrons, a sort of follow on from the ATC. These can only be described as awesome fun - lots of flying, which you get paid for, the best instruction, and some great fun to be had. I know this from personal (current!) experience.
Things to avoid: rushing into things. ALWAYS plan things: don't expect to pay Oxford Aviation (or similiar company) 60,000 and expect a job straight up. Look at all your options (including whether aviation is right for you). Perhaps the best advice I can give you is to do the GAPAN aptitude tests at Cranwell ( google the word 'GAPAN'). This will give you the best indication of your suitability towards professional aviation as a career, and they will also give you more good advice. Lastly, check the CAA website for the medical standards required for Class 1 certification; important to make sure you can pass these important hurdles!
Of course, there's a lot more info to be had. Feel free to PM me, and look around the rest of this site. There's a LOT of useful info to be found in these hallowed pages!
Eoin