Alternatives to authorize.net?

As a UK user who has set-up UberPOS, I can't use Authorize.net for UK credit card transactions. It's a shame because this seems like a real viable alternative to OpenBravo POS, especially given that my recent background is Drupal.

Can anyone explain how easy it would be to write some code that allows use of a UK friendly credit card / payment processing system for UberPOS? It really is the difference between using drupal as a POS solution, and going somewhere else.