Can anybody recommend a decent integrated payment gateway / API service for UK Zen-Cart users that does not require customers to leave the website?

Paypal Payments Pro looked like a good option, but I called Paypal and they said that it will be 3 to 4 months before this option is integrated into Zen-Cart.

The other (expensive) option as far as I can tell is to buy a real credit card machine/link and use the basic Zen-Cart credit card module.

Any ideas?