Forum Moderators: buckworks
For the U.K. company, you could look into WorldPay, MultiCards or Metacharge. I know that MultiCards is compatible with OSCommerce.
For the U.S. company, I would recommend to open a direct merchant account as there are many options out there. Also depends on sales volume. If you are looking for a third party processor, 2CO, CCNow and Paysat come to mind.
Possibilities are probably endless, but these are the ones I always suggest as they are trustworthy and have been in business for several years. (except Paysat, but I have heard good stories about them.)