Add Themed Shopping Cart Facilities to Wordpress Site

We need to add shopping cart functionality to our Wordpress website. We would like this to be themed according to the existing site theme - www.thseterlingdatagroup.com and we are open to advice concerning payment processing solutions.

Solution needs to provide us with the ability to quickly and easily add products (with optional services), images and descriptions.

One stipulation is that any cart/payment processing by a third party must be by a UK company.

We would like this to be be up and running by Monday if possible.