"I am 18 and half way through my application for the RAF to become an Air Traffic Control Assistant.
Currently, the RAF are "fast-tracking" Assistants to Sgt (and therefore Controller) in 3-5 Years."
They were saying the same in 1989 when I joined up, in reality it didn't happen that often, maybe it does now, but I've been out for 8 years now so can't be sure. I now work for NATS as a controller. You'll gain lots of experiance, more than most current civil ATCOs who think they know everything but they don't, but then you will still have things to learn (as does everyone including the ones who have been doing it for years).
The RAF is a funny place to work. As CM said, faces have to fit (as it does with NATS), you might be the best in the RAF/company but if your mentors/bosses don't like you or if you don't go out for a drink every now and again you're stuffed. In fact being the best could mean you're stuffed coz some out there don't like you to be better than them. One of the weirdest things that used to happen in the RAF was that you tended to get promoted to a controller if you worked in Ops, but not if you worked in the tower (they were the same branch then). This was because your annual write ups were done by pilots who didn't really know you and couldn't be arsed to do a proper report, where as in the tower it was done by the people you worked with.
Anyhow good luck with it, whatever you do you seem to love the job and not care how you get to do your dream