Your depth and breadth as a person will be more important than any amount of 'spotting' knowledge.
Don't concern yourself with AC/UAS's etc, it provides no more help than showing you how to wear the uniform. "Die hard life long wannabes" usually remain wannabes for various reasons - some medical, some professional, some just to do with your face not fitting.
Granted, you'll be required to 'know your trade' but that goes for any job interview - you would not wander into anything else without having done some background reading.
Have the courage of your conviction, know why you want to be there and make sure you communicate this when asked; and for god's sake don't say anything about 'Queen and Country' - they'll see right through that.
Consider going to university, or at least talking to a Careers officer about it first. Remember you'll be up against many people who have, and while you're there (the careers office) also consider what you want to fly - every man jack goes straight for the fast jet option, so again think about the competition.
Good luck - read up on your career options, prepare mentally and get fit.