2 speed is little derailleur thing. 3 speed is hub gear (a kilo or so heavier). 6 speed is derailleur + hub gear.
2 speed is fine around london (and a bit lighter). Ergon grips are a great shout. Carradice bags are very nice if you get the luggage block thing on the front.
So I'm getting a brompton through cycle to work scheme. What do I need to know?
Had planned to go to London fields cycles and try some out there.