SLJ
To try and prevent you being mauled by some posters - we don't train new navigators anymore so that path will not be an option. This branch may be of interest as below.
Weapon Systems Operator - Aircrew jobs - RAF Careers
The only time I flew in the Lynx in Iraq as you describe, the lads with the machine guns out of the door were British Army. I'm pretty sure crewmen/women in the RAF have done similar but I've never seen it so don't write with knowledge.
Go and have a chat at the RAF careers office closest to you for the most up to date information. Good luck.