Dracfromloc doesn't mention the need to take the folder on a train or bus, so maybe he doesn't actually need a folder at all. I think you normally expect to pay a bit extra for a bike that folds because of the additional complication of making it and the fact that the niftiness commands a premium.

There is no doubt that quite a lot of folders with a reasonably average rider can be commuted for 12 miles. How sweaty you get depends how fast you want to go. You might also pay attention to the likely winds and bad weather as well as to how flat the route would be. Then there's the question of how traffic free the route is. There's not much worse than riding along the edge of a fast busy road being blasted by the slipstream of trucks and inconsiderate drivers. I nearly always seek out pleasant quiet routes, but it isn't always possible. That's when I chuck the bike in the back of the car and join the other motorheads.