I had a very careful read of my cadet training contract when I got it and came away with the conclusion that, according to the contract, BA could force you to pay back the full costs (thats £60000!!) if the training was terminated. This is irrespective of whether the training was terminated by BA or the trainee. Basically the contract covers BA for all eventualities.
The only event in which BA cannot recover the costs from you is if they do not offer you a job within 36 months. As to the comments relating to offers of employment as a cleaner, they are correct in claiming that the contract refers only to an "offer of employment", although it does refer elsewhere in the contract to "employment as a pilot". Six of one, half a dozen of the other. I think that you could argue that the offer of employment is implied by the nature of the contract to be an offer of employment as a pilot and if that is not the case then the contract should explicitly say so.
Away from all this legal chicanery and back in the real world, the situation is this: BA are well within their rights to ask for the costs back if you leave the course or even if you are chopped. In practice this does not happen. One guy was chopped after leaving Oxford and walked away with a frozen ATPL and no debt. Another cadet decided quite early on that BA and piloting was not for them and left. Again, no debt.
I think as long as you are honest with BA and try your hardest then you needn't worry about a £60000 debt. It's not in BA's interest to have people working for them who don't want to be there and there is little point in chasing a student for £60000.
Failure rates are extremely low due to the selection procedure and the determination of almost all the trainees so thankfully this is a debate that rarely has any practical application.