Originally Posted by jimcbob

The payment is not the key point. The contract will be formed when BA and the customer, objectively, reached agreement to vary their original contract. That would be, I would have thought, the final stage in the online process where the customer clicks "Confirm" or "Make this Change" similar, and BA.com spits out a message saying that the change has been made. Looking at it the other way: once the customer had clicked the button, and BA.com had accepted it, could the customer insist in reverting back?

Amazon, etc. work slightly differently because they have specific terms that regulate when a contract is made (usually when the products are dispatched and payment taken, which is why you can 'cancel' up to that point) to give them some more flexibility.