Originally Posted by O27PMR
As far as the military route is concerned, you need to be of a certain educational standard and age:
RAF - British citizen, Under 23 with 5 GCSE's (grade a-c inc. maths & english) and 2 A Levels
Army Air Corps - British Citizen, Under 26 with 5 GCSE's and 140 UCAS tariff points at A/AS level
Royal Navy - As AAC, not sure about the age limit though.
Slight correction: you need to be under 24 for the RAF, under 26-and-11-months for the AAC (although they make exceptions), and under 26 for the RN (although you must be in the navy by 25). If you want to fly helicopters (as opposed to fast jets, etc), the AAC is probably your best bet, however you need to go in with a comission (i.e. as an officer) to go straight into flying. As with many things military, it pays to start as early as possible. If you have any particular questions about the forces, feel free to PM me, and I'll do my best to answer - I'm not an authority, but I have done some research into joining various branches.