I owned a bicycle shop for nearly 45 years until I sold it to my long time manager and retired end of December 2015. We sold a lot of Bromptons. Excellent folding bikes and a very loyal following. Pretty much custom order from the factory in England placed through your nearest USA Brompton dealer. They are not inexpensive. But they fold smaller than any other folder and still offer a good ride. Every customer that bought one loved them. We sold them to long haul truckers, apartment dwellers, small plane pilots, boat owners, etc.
Biggest issue is rust from the salt air (true w/any bike). It kills me to see a bike stored on deck under a tarp. You have to show them some love and attention if you want them to last.
Take a look at folders from Giant Bicycle. We sold a lot of them too - aluminum frames and fairly common parts. (Bromptons use a lot of special parts - but then they don't break down very often). Giant aluminum frame - that helps ward off some of the rust issues.
DaHon also makes folders. Years ago they made a Mariner model - aluminum frame with a lot of stainless steel parts. Good deal if you can find a used one.