sljmaster,
If you want to do nothing more than hang out of the side of a helicopter on the end of door gun, then you probably need to look at joining the AAC in the ranks. Once you have done your basic training and have a tour under your belt, you can apply to be a Crewman / Door Gunner. But you may only do that for a tour or 2 tops, it isn't a full career stream.
If you want to do more than that, then as has already been you need to look at Weapons Systems Operator in the RAF, but in doing so, being prepared that you could end up working on the Air Transport or ISTAR fleets as much as the Rotary fleet. It all depends what we need at the time and where your aptitude lies.