For the United Kingdom, you can try either WorldPay or FastPay. WorldPay is backed by The Royal Bank of Scotland while FastPay is backed by NatWest. So both have very good financial backing.

Authorize.net is good, however you require a U.S.A. residential address as well as an American Bank Account, as far as I'm aware. However Authorize.net are very very good, apart from those side parts.

PayPal - Don't use, if you respect your business. I find the majority of children signup with PayPal. Accept PayPal if you don't care whom signs up, although if you want to be known as a respected host who doesn't host mainly children, don't use PayPal. Also, lots of people see you as being a child by using PayPal; due to the fact, although there TOS and AUP say no people under 18 are aloud to signup, many still do which is very unfortunate. Therefore, go with a real merchant account or a 3rd Party company, and only accept Credit Cards pureply.

Also PayPal isn't good since your clients need to have a PayPal account. However, they don't need one to signup, since the signup process creates them one, although this can still be very annoying. If I walk into my local super market, I don't want to open an account with their processor merchant just to buy a pineapple or a packet of frozen veg off them. Do you?