As the amount was over £100 you should be protected. This from moneysupermarket:
It's well worth using a credit card for purchases of £100 or more because you have greater protection than if you pay with cash or a debit card. Under Section 75 of the 1974 Consumer Credit Act credit card issuers and retailers take joint responsibility for faulty purchases. This applies to purchases between £100 and £30,000. If the retailer goes bust or the goods are faulty when they arrive you can claim a refund from the card provider.
I can't find anything on it being illegal to use a CC for online gambling. Probably because of the usual 'international laws' problem of someone using their computer in one country and the website being in cyberspace.