You'll have to go to Cranwell to do the aptitude tests anyway, so apart from giving you some practice at them, I can't see how paying for the GAPAN tests would help.
If you're trying to show a commitment/passion towards aviation, I would have thought that paying for a trial lesson on a light fixed wing aircraft would be a better bet. If you like the lesson, then talking about it in subsequent interviews will be a useful way to get across your passion for aviation. I had a fixed grin on my face for days after my first flight
Just be honest about it being a recent interest, dont get too hung up about not joining straight from uni nor applying for sponsorship. The Navy did run a whole recruitment program a few years back with pictures of pizza delivery boys/warehouse stackers/bus conductors saying that they would have been harrier pilots/missile loaders/warfare officers in the Navy, so your comments about industry may strike a chord with your interviewer. Just be honest about it as they will spot bullsh!t a mile off.
Dont get hung up on the number of applicants you are up against, somebody has to get the job and there is no reason why that cant be you.
Good luck with it, I hope it works out.
