Back to the point
Just to get back to the point of the original thread, there's no reason why you shouldn't aim high Sainty, but you've got a massive uphill struggle ahead. Unfortunately for you, there are plenty of kids out there who are sh*t hot and brainy as anything who nailed their qualifications first time, and a lot of them would kill for the job. Basically you're going to be at a bit of a disadvantage because of your age and that you failed to reach the standards first time. But that by no means makes in unachievable.
If you decide to do it (and my in my limited experience I'd say the job is awesome) you'll really need to prove to the RAF or RN that they need you.
Noos