Are there any alternatives to the OpenSRS which, like OpenSRS, offer the ability to put a version of their domain registration and modification forms on your own site.

I have not got my Hosting Reselling business off the ground yet, and have no idea how many people will be registering through me.
I would rather pay $15 or more per domain without having to pay so much up front.

Ideally, I would also like a registrar which supports co.uk domains.

Any ideas....?

Thanks, Simon.