If it's a UK company you're entitled to a FULL refund under the Consumer Contracts Regulations:
http://www.which.co.uk/consumer-rights/regulation/consumer-contracts-regulations
If it's an overseas company the laws of the relevant country will apply. (Remember that many traders in, say, China, try to look as if they're UK-based by using .co.uk web addresses. So don't assume that it's a UK trader unless there's a valid UK postal address on the website).
If you use a CREDIT card [i]for a purchase of over £100[i] (and under £30,000) the card company can be held liable (under Section 75 of the Consumer Credit Act) for any failure by the supplier.
http://www.which.co.uk/consumer-rights/regulation/section-75-of-the-consumer-credit-act
However the rules do NOT apply to purchases of under £100. Neither do they apply to DEBIT cards. Some card companies might offer a 'charge back' arrangement but that's entirely voluntary and you've no right to insist upon it (unless, of course, it's clearly stated in your contract with the card provider that they'll guarantee you a 'charge back')