The UK military flying training system is 100% geared towards students with no previous experience. The aptitude tests at OASC do not test flying knowledge, they test for a base of abilities in numeracy, reasoning, information handling and co-ordination that have been shown to be needed before EFT commences.
You are far better off spending your time getting fit, and going gliding if you want to get airborne. Powered flying is very expensive in the UK, and really a waste of money if you are hoping to join as a pilot.