In reply to Jamming Dodger:
I used to have a Dahon, now have a Bromton. The Brompton is a class above the Dahon in my experience. I find it a better ride (bike is stiffer), and the fold is way more convenient. The Dahon wouldn't really fit on all the train luggage spaces, but the Brompton does.
Personally I'd say go for the 3 speed. It's a sturmy archer internal. The single speed isn't really much good for distances, the 2 speed is a derailieur and the 6 speed is just a combination of the two, with 2 gear changers.
If budget is no option, then consider the Titanium frame over the steel. Not so much for riding but for when you have to carry it. They are not light. Really just good for carrying onto the train/putting in the boot of the car, you wouldn't want to carry it for far.
(yes, the Brompton does fit in the boot of my MX5...just)