I'm around 5'7, maybe a little shorter and going through army officer selection. I was the littlest in my group but didn't really affect me. Smaller legs means you'll have to work at running a little more, but everyone has their own weaknesses so be thankful yours is something that doesn't matter. Don't know how selection works for RAF but I'm sure there will be tasks when you need smaller people. Just work as a part of your team and don't stress.
If you're worried because you think you'll be taken the **** out of, you've got plenty other of things to be worrying about that matter more than just being a midget.