"is it possible to train as a gunner or some thing and then train as pilot at a later date or not?"
There is no basic reason why, if you join the RAF Regiment in 'the ranks' as a Gunner, you should not eventually apply for commissioning and pilot training. Provided, that is, you pass selection for the Regiment in the first place. See
http://www.raf.mod.uk/careers/jobs/r...mentgunner.cfm
Arters, actually I think you'll find that there are quite a few door gunners on SH aircraft these days.