A lot depends on what educational qualifications you have got. If you haven't got the GCSEs then you can't become an officer, whether direct entry or through the ranks and, therefore, you can't become a pilot in the RN and (I'm pretty sure) in the RAF. As has already been mentioned, the Army are a different kettle of fish.
While being very keen, achieving gliding and powered qualifications is laudable and will do you a lot of good in the selection procedure (assuming you have the GCSEs), gone are the days of being able to slip in through the back door on the basis of non or pseudo-military qualifications. I'm afraid there are a lot of people out there who do have the educational qualifications AND they have their PPLs etc and the armed services can afford to be extremely picky nowadays.
This is not meant to be a message of discouragement but one of realistic expectations in today's forces, not those of the 1960s. You've got time on your side so go back to school/night school and get the GCSEs if you don't already have them, or else you'll never pass "Go". Definitely keep up your keen approach to the ATC, which is an excellent organisation, and if you can afford to get some flying in, powered or non-powered, then do so....it'll only help in the entire process.