Can a Yank throw in 2 cents?...
I spent 13 years in the military, and I truly loved it! I wasn't an aviator in the Army, and I think that is one of your concerns. Even if you don't get the position you want initially, you will gain valuable working experience, build a solid record, and will have many benefits as a veteran... not to mention the opportunity to see and do some very cool things!
I didn't start flying until I became a civilian. I would have stayed in the military for 20 years, but I had a few financial opportunities that I couldn't pass on from the outside (became a computer geek and couldn't afford to stay in the Army).
If you decide to join the military, sign up for the minimum specified time allowed for the position sought. Military life isn't for everyone, so if you decide it isn't for you - at least you won't have a L - O - N - G committment ahead of you. I don't know how the U.K. military is structured very well, but in the U.S. you can still apply for WOFT (Warrent Officer Flight Training) slots from any job field.
BEFORE YOU JOIN - Make sure that you can pass the required medical (if flying is the only thing you want to do). Eyesite is a big one. Although "waivers" may be available, don't count on one (they are usually hard to get). Good luck!