Hi The Cribbys, what you are saying is very interesting indeed.
I know BA can change its mind and can offer you a temp contract and / or other fleet other than what you might have applied (and been interviewed for) in the first place.
However I would have thought that when you are offered a training date you would also be told what airport and fleet you'll be allocated to, even more if they are unable to send you the contract before the training start.
Personally I would never start a training and let alone leaving my job if I don't know exactely what I am in for, and I would expect that any reputable company would not expect me to do otherwise.