The chain tensioner is an essential part, even for Bromptons that don't have the 2-speed external derailleur (i.e. the 2-speed and 6-speed). This apparatus keeps the chain taut both when it's being ridden and when it's folded, so it is necessary on both the 1-speed and the 3-speed models. Instructions are included with the parts and they're available here.
Manual / Data Sheet