I've taken a look at the PayPal Commerce page and it is an API for accepting payments to multiple PayPal accounts in a marketplace. I don't see why the UK/EU would need to switch to this in order to take payments on Zen Cart. Probably your best bet is having a module written with the Rest API or Braintree SDK. We already have a basic module for Braintree but it doesn't have the options for PayPal, just credit card. That could be expanded though.