No, let's say I bet £100[0] on an exchange. That doesn't guarantee the bet will be matched. Maybe only £10 of my bet is matched. And it's the counterparty or co

No, let's say I bet £100[0] on an exchange. That doesn't guarantee the bet will be matched. Maybe only £10 of my bet is matched. And it's the counterparty or counterparties to the bet that are on the hook, not the exchange. I.e., it works much like a marketplace for other financial instruments, and the situation I'm describing is much like a "partial fill" on a stock order. The exchange takes its commission on the matched value of the bet no matter what, and they don't care which way it goes. [0] I'm using small numbers because the total value of matched bets on many horse races can be small. If you go in and drop £100k or £10k, or even £1k, especially if you did it all in one go (and assuming the exchange would allow it), there are plenty of races where only a tiny portion of that would be matched. In fact most races have only thousands of pounds or low five figures matched.