as well as splashplastic, there is
http://www.mycashplus.co.uk/
i can personally vouch for mycashplus as i have one in my wallet and used it to buy xim. it is a pre-paid credit card so you can only spend what you have put onto it. it is a Mastercard card, so its a real card too, it can be used anywhere, online, shops, cash machines etc.
i have one and i highly reccomend it if you dont need a "real" credit card.