Join as AAC groundcrew, and once promoted to Lance Cpl apply for aircrew.
Aviation Crewman - British Army Website refers.
I'm guessing that there are no min reqs other than passing the recruitment tests administered in the recruiting office. GCSEs in Maths and English will help later, though. Note that the Air Gunner does not get to fly the aircraft - it's a Lynx-only role which will normally fly with two pilots and one/two AG.