With your existing experience and RAF qualifications, I'd have thought you could get into a good BEng/MEng course without much difficulty. Also given those, anything more than a BEng would probably be wasting your time - you've already got a lot of specialist and working knowledge. So, 3 years - which is, admittedly, still a lot. I've been whingeing for years (and despite a few hopeful starts, still getting nowhere) about the lack of a part time / distance learning first degree in aerospace or mechanical engineering.
As for what course to take? - the main courses (the names change a bit from time to take) you'll come across are:
Mechanical engineering
Aeronautical engineering (also called aerospace, aviation...)
Electrical engineering
Electronic engineering
Systems engineering
The aerospace industry (and most other branches of engineering) is full of graduates of all of these. With your RAF experience I'd not worry much about employability - the problems are all for graduates without experience.
So, my suggestion, if going that route, is pick the one you think will be most fun. Mechanical if you are seriously into "hardware"; Aeronautical if you are into aerodynamics, whole aeroplanes, ops; systems if you are interested in the control and integration of engineering. Electrical and electronic should be obvious.
G