As I understand it on the charity/pledge thing, if the charity 'sells' the place for a guaranteed sum (say, £1,000), then the £1,000 is not a donation so it's taxable.
So, if you sign a contract agreeing to give them £1,000 for their marathon place, you are bound by it, and they could pursue you through small claims court of some such for the money, but they would have to pay tax on it (and probably incur some negative publicity along the way).
The wording of the contract may be a bit more ambiguous, though, in order to get round the selling issue. Then the contact is less likely to be enforceable, too.