There are plenty of mail forwarding services that RVers in the U.S. to choose from. One of the most popular ones is through the Escapees RV Club and is what I would probably use if I needed it. When we lived full-time on the road, our mail forwarding service was my mother-in-law. She would send us a box once a month, general delivery, to whatever town we lived in.
I just Googled mail forwarding services in the United Kingdom, and it definitely looks like they have options. Maybe it is too expensive and when you are living like a gypsy, budget is a big factor. Hmmm.....