Nick & Joe,
Firstly, I wish you the very best of luck with your applications. I too believe the Fleet Air Arm is the way to go.
I've (very) recently been through this process with the FAA myself, and managed to get selected as a pilot in spite of my age (now 25), and also in spite of a low AIB score. Selection is alledgedly on a 'competitive basis', with the highest score in the pool of eligible people (i.e. those who have passed FATs and AIB) being selected first, so I can only conclude that the competition must be very strongly biased towards aptitude scores. This just goes to show how important it is to do as well as you can on the day. (No pressure or anything!)
It is true that some of the tests are impossible to prepare for, but not in all cases. I prepared very thoroughly and it paid dividends. I would suggest:
* Practice quickfire mental arithmetic (e.g. +,-,x,/) with 1 and 2 digit numbers - no pen and paper allowed.
* Practice speed-distance-time problems, e.g. "aircraft A sets off at 1100hrs from base A towards base B 200 miles away at 360 kts. 10 minutes later aircraft B sets off from base B towards base A at 300 kts. At what time, and how far away from base A do they meet? - this type will be multi-choice and pen and paper are allowed. (not all as complicated as that though). If you don't know it, just move on. Incidentally, any common errors that you might make, e.g. forgetting to add the 10 minutes, will more than likely yield an answer matching one of the other incorrect choices.
* Practice remembering long numbers. I leave it up to you how you do that. The numbers will get longer and you will get marked on how many individual digits you remember, not whether you get the whole thing right, so when you get to your limit (e.g. 9 didits??), don't risk remembering any more - just cash in what you can remember.
* Be familiar with basic aircraft flight instruments, especially artificial horizon and compass. You will have to match instrument readings with pictures of an aircraft in a particular attitude, and also later on match a larger set of instruments with a verbal description, which will include speed, heading, attitude, climbing or descending, etc.
There's a lot more to it than that, but I don't think you can really prepare for any of the other tests, except by getting a good night's sleep the night before. I don't think a pint or two in the bar will do you any harm the night before; it will probably relax you a bit - just don't get too relaxed, because the beer comes at real mess prices. (Cheap!!!).
Once again, good luck
Pie