In all honesty, competition for a VI Form Schol is so intense that it's not really worth putting down more than one choice.
When I applied for one [2001] they only gave out six for the entire year, both pilots and engineers.
Just say you want to be a pilot. If they pick on you during interview - or rather, when they pick on you - just say that you are quite singleminded right now, but if you were unsuccessful your subsequent attempts at OASC may involve a wider range of Branch choices, depending on why you were unsuccessful this time.