@helena
The selection process is oriented towards you as a person rather than towards your technical knowledge. They work on the basis that you'll be taught what you need in training anyway, though having some background or previous experience would of course assist the training stages.
Being female would not disadvantage you in any way with BA. In fact I think my entire selection process was only ever carried out with female staff and female pilots.
@alosaurus
First time passes isn't a requirement. As long as you have the required grades then you'll be fine. I can't recall if they're entered on the form or not, but your certificates would be photocopied at selection and mine were never mentioned again. Resitting exams could even be turned round into a positive indicator of your desire and determination.