Um, well, naturally, you have intentions of working on N registered aircraft.
You say you are in the RAF working airplanes, so you have a training record, so challenge the test, just take the dang thing.
The air ledge will kill you, so a course there would probably help, no, wait, it will help, but the basic stuff you should already know. I don't believe you really want to take a course on how to safety wire or dope a wing or balance a wood prop or any other number of mundane things that you probably know, so why don't you take a peek at the test/qualification requirements and go from there?
You may be surprised and save yourself boo koo bucks.
Check out this link:
http://www.faa.gov/mechanics/become/
Good Luck!
PB