You can get a pre-paid Mastercard / Visa from many sources like Western Union. Actually, you can get an American Express Gift card if you want too. There are many sources for these, just make sure you get one from a reputable dealer like Western Union or AmEx and pay attention to the fees.
What's wrong with WU? I've never had a single issue with them, nor has anyone else I know. Maybe they're bad in the UK?