Akistair, welcome to Pprune and thank you for your post.
Sponsorships are few and far between and very difficult to secure. BA is not sponsoring anyone currently, and they have made no announcements for the future. It is possible that they may have reintroduced a cadet scheme by the time you are in the market, but it seems most likely to be similar to the CTC scheme which involves you signing responsibility for over 60,000 pounds worth of debt.
While you should never reject the possibility of obtaining a part-sponsorship (like FlyBe or Britannia) or deferred-payment sponsorship (like easyJet through CTC), you have to accept that the vast majority of commercial pilots these days pay for their own training. There is a great deal of information here to help you prepare for that.
If you decide that the military is your bag, you will have to think carefully about what A-levels you take and whether or not you go on to take a degree. The RAF, Army and Navy websites will give you a great deal of information about what you should do.
The best thing you can do for now is take time out to study other peoples' stories here on Pprune, and try and relate them to your own circumstances. Once you've got a feel for what other people have done, you can start to ask targetted questions here.
Best of luck!
Scroggs