I'm guessing it's roughly the same as a shop pricing an item at a clearly incorrect price. In Law, the shop does not have to sell anything until both its representatives and the customer agree to the sale. In Betfair's case, since they accepted the bet, it would be interesting to see how they could wriggle out of it, other than to say it was in their terms and conditions. They, themselves, could be challenged under The Unfair Terms in Consumer Contracts Regulations 1999. Interestingly, Gambling Debts were unenforceable until 2006.