We are Americans who have visited the UK many times, and have been to all four of the component kingdoms. However, we wanted to visit the out of the way, difficult to reach places that not even most Brits have been to.
I had never heard of CMV nor the Marco Polo, but surfing on line for the itinerary that I was looking for, I found the "Around Britain" cruise that met my criteria. The port calls were Invergordon, Stornaway, Tobermory, Dublin, The Scillies, Guernsey, and Honfluer.
I also discovered that CMV is attempting to attract more American and Canadian passengers, and has opened a sales office in Florida to deal with them. We booked the Around Britain cruise departing July 26 with them. We paid entirely in US dollars and all went smoothly.