Go to your local AFCO, same as for Crabair

There you will find a plethora of valuable assistance, although most of it to be honest from pamphlets

Find out from either reading up the pamphlets or looking at the careersy bit on either the RN or RAF websites and make your choice.
Once you go into whichever AFCO, do not let the person you speak to try to talk you into joining up as something you don't want to do or you WILL regret it forever!! He/she will tell you anything to get you to join up as a whatever they need at the time, even telling you that its easy to change once you're in, which it damn well isn't.
Hope this is more helpful Katie