I see you are interested in Vintage - that is very good. Newer Airstreams 25 feet long and over are 8.5 feet wide, older ones are 8 feet. It makes a big difference if you get on country roads in Brittain! Also you'll probably not want to go super long - 26 feet INCLUDES the tongue. I think you've targeted a good maximum length there.
In addition to electrical upgrades, there are shipping costs. One member here "Tin Crumpet" and her husband bought an Airstream Motorhome - on our West Coast. They actually came over here, and drove to a southern port in Texas or Louisiana and shipped from there. I think that actually saved them money over shipping from the west coast.
Oh, and don't forget CANADA. There aren't a whole lot of Airstreams there - but they might be wired more in line with the British standard. And if you imported from the within the commonwealth - might you avoid import duties?
It might be possible to find one in the U.S. and get someone to transport it to say the Canadian side of Niagra Falls - then sell it to you. Duties? Well no one wants to do illegal stuff, but legal tax avoidance - sure.
Anyway, happy hunting - and have you found our "classifieds" yet? A couple of good East Coast ports are: New York City, Baltimore MD, Norfolk VA, Savannah GA. (BTW - I'm right on the edge of Norfolk.
Happy hunting, Paula
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