Speak to the Senior Air Movements Officer at Brize Norton.
He's got a list of costs etc, and will be the ideal man to ask with regards to the possibilities of getting kicked off at the last moment.
I enquired as to the possibility of indulging (with Mrs in tow) to the FI about 5 years back. I was told that I would need to prove that I had the money in the bank to pay for a civvy ticket back, x2, via Chile/USA in the event that I was kicked off the flight penguin side.
I decided not to bother in the end as the tickets back are a grand and a half a piece. The day I have that sort of cash floating in my current account is the day I don't need to indulge anymore!
(By the way, cost to indulge was, back then, about £150.)
Hope that helps.