Interesting that they might rebook you onto another carrier. I would have thought that they wouldn’t even consider this. I would assume that you would have to collect receipts for additional cost and submit them afterwards. If they refuse to rebook you can always book yourself and try claiming it from them. I would think that chances of a successful claim are rather low, but I might be mistaken.
There seems to be a flight on the 23rd with Jet2. Maybe that’s an alternative (according to this snippet you posted they have some sort of agreement with Jet2)? Otherwise, you could ask them to cover the train fare to London and take a flight from there.