If your a British citizen and planning on going to Uni in England then I suggest you join the University Air Squadron in your first year. It is still a way off for you yet, but bear this in mind as it is the perfect way to start your career in aviation and doesn't cost a penny. In fact you even get a small bounty for your 'troubles'!
If you manage to get in (there is a selection process, but don't be put off by this as it is good experience either way) then it will be down to you whether you want to join the RAF after graduating or move onto a commercial airline / instructing / general aviation etc..
Best of luck!
Jim