Luke,
You might get more detailed responses from the Military Aviation forum. But, to answer a couple of your questions;
Between 13 and 18 there should be a local Air Training Corps Squadron which you can join for some good ideas on what to expect from the RAF and vice-versa. You normally need to attend two evenings a week. As a rule of thumb, maths and physics will be required as A-Levels for selection to interview, amongst many other personal qualities. Additionally, up to 70% of new pilots now come from the UAS (University Air Squadron - free flying training from the RAF while you study a degree) so a university degree can actually HELP your application, as well as provide a backup.
The first thing to do is look in the phone book for your local RAF Careers Office and phone them for information.
Most importantly of all - be yourself and good luck
Andy