While at Uni, whatever you're studying, join the University Air Squadron (or whatever they're calling it these days). You'll soon find out whether you're fit for the RAF and by extension the other Forces. Either way, make your mind up and do it fast, or the opportunities will have passed you by and with some opportunities there are no second chances.
I developed stereoscopic vision this summer. At 32, it's come far too late for me to even think about join Her Majesty's Flying Club. The idea that my not having done so is because I was just not trying hard enough is a real slap in the face, given that flying was all I ever wanted to do from the first time I looked up. I will learn, I will fly, even if it takes me another thirty years - but I'll be paying for it with money rather than with service.