Ask HN: Can you reccommend a good online card processing service for the UK?

I'm building a site for selling audio downloads. I expect a low volume of sales, initially and therefore prefer to pay per-transaction. I'm planning to use the Active Mercant rubygem. So far, nochex looks like an option that would work. I've looked into Paypal express, but they're not native to the UK, and they charge extra for currency conversion and cross border fees.