As for "international license," unless things have changed recently your U.S. state-issued DL is good for tourism rental/driving in UK. Just have to get used to that other-side-of-the-road business. . . Good luck with the planning and have fun, wherever you end up!

Not been to Gretna Green but had a potentially analogous experience on an early UK trip when we planned a stop at Cheddar Gorge. Drove straight through when we found it overrun by tat shops and tour buses. . .