Tiger
From what I can see, no expert though, you are beyond limits for Class one and military. As far as I know the RAF require 20/20 uncorrected vision when you join. I do know though that for a class one you require - 3 dioptres in both eyes until 25 then up to -5 at initial. I then think you can deteriorate to -8. Under new JAA rules which have been accepted in theory you can be up to -5 at any age. The difference between eyes must also be less than 2 d. However you still don't meet this. I would suggest you try Lasik as you are a little too myopic for PRK. You have nothing to lose. 'Down' time is a year so plenty of time.
Sorry to bear bad news.
SK