I did flying training for pilot in 1986 but ultimately failed the Gazelle course. My advice is pretty valid, however, as I now see a lot about where I went right and wrong.
First - do you really want to do this? I loved flying and really knew it was what I wanted to do, and also knew I had some aptitude for it, having been in the Air Cadets and flown a fair bit. You are going to have to put a hell of lot of effort into this course, so be sure it's your bag. Go and do an acquaint visit. Talk to some aircrew.
Make sure you are ready to join up. This was my failing, as I joined after A-levels and was more than a bit green. A few years after, I suddenly woke up one day (having just spent four months travelling across China on my tod) and realised I was now the guy they'd been looking for when I joined - no coincidence that most of the guys on my course were 22-24. Of course with most aircrew being grads now, that's not so unusual.
What kind of person are you? You are going to need to be pretty good academically - although, I have known some great aircrew who had little more than 5 O levels or a couple of A-levels in funny subjects like History or English. That doesn't mean they were wooden, though.
Forget being an observer unless you are very good at maths. Pilots can get away with a good basic maths level (I got by with O level), but Observers really have to have a pretty sharp brain for figures.
Also, naval aircrew are team people. If you are a loner who doesn't like working with other guys, forget it.
I'd say most aircrew are a little bit, let's say, special in some way. I don't mean they are uber-humans, or anything like that, but when I was at Dartmouth the Captain there once said he could always tell the aircrew courses when he was looking out the window. Probably because we were winding up the parade staff or having a laugh, but also, probably because we were the fittest or smartest.
That's the kind of bloke you have to be. I now live in Russia and work in a news agency, having learnt Russian. Another guy in my course now runs a drumming school in Gambia (no, really!!) Quite a few of the boys are airline pilots of course. Not the average.
Things like the survival course, or doing the dunker with AR5 on in the dark, do tend to stretch you a bit.
Its' the best job in the world, and I still regret not having cut it. Good luck, and remember, if you can't take a joke....