In common with what everybody else is saying, the RAF and its affiliates can help you enormously - I would truly recommend joining the ATC, even if you have no interest in flying for the RAF and only want to fly commercially. Through them you can get your flying scholarship, but more importantly if you want to start doing some gliding, you can get that for free. When I was 16 I got a gliding scholarship through the ATC and finished up with about 10 hours, all free and all good training.
As it would appear you are already doing, continue to immerse yourself in everything related to aviation. Airlines now want to see potential pilots being very business aware and by just looking at Pprune, you're giving yourself a big advantage.
A-Levels wise, most sponsorships want to see evidence of Maths and a science based subject (Physics or Chemistry) and it wouldn't hurt to throw in a Business/Economics A-level for good measure. That being said, I chose Maths, Physics and Business and looking back, wish I'd chosen something I'd enjoyed as I would have done better! I've got away with it, but most airlines place a huge amount of emphasis on academic qualifications, simply so they have a way of screening people out from the huge number of applications. I would recommend going to all the airlines/groups that sponsor and getting details posted to you (CTC, Britannia, Gapan etc. etc.). After all, at some point they're the ones you'll need to satisfy that you are worth the investment.
As has been said to others in other threads, its well worth considering your options regards university. If you're adamant you're never going to self-sponsor, then going probably isnt a bad idea - mainly because you don't close off doors to other career paths should the worst happen and it not work out. However, if you're going to pay for your ATPL then it might be an idea not to spend the money on Uni as it probably won't help you enormously (although thats not to say it would be useless!).
Finally, just be yourself - spend as much time dealing with all things that are industry related as possible. I'm starting my training in Jerez in 6 weeks time and then coming back to a job. Up to this point, I've spent time in BA Engineering on work experience, worked on the ground for BA (Check-in, boarding, shadowing dispatch, ops overview) and worked as Cabin Crew. I feel that all this is very valuable to me as it gives me a better perspective of the 'bigger picture' and I'm sure it will help when I come back as an F/O. Take every opportunity that you get and I'm sure you'll be absolutely fine.
Best of luck to you, it'll be tough but oh-so-worth-it!
FJ2k
