Bear in mind that you are really only competing with yourself - it's not X-Factor, and the assessment is done to ensure that the RAF gets the people best suited for the job. While preparation is vital, they are not really interested in whether you've wanted to be a pilot since you could walk - they are taking a snapshot on the day, not measuring (if it were possible) some hard-to-define and long-felt desire.
The other aspect is that some of those who are ex Air Training Corps and/or UAS may actually think they deserve the job, and can come over as slightly arrogant. That really annoys the interviewers.
Remember the old phrase - Prior Planning and Preparation Prevent Piss Poor Performance.
Good luck!