Brompton has a 2 speed freehub, 12 & 16t cog, Chain Shifts by pushing the chain tensioner pulleys back and forth, and they slide to accommodate, sideways.
rather than a vulnerable RD sticking out. a clever method..

part of the small fold is narrow dropouts on both ends 110 ish rear 74 front

people have fitted other IG hubs on Bromptons. SA 8 speeds are one of the choices..
Nexus another ..

You can narrow the overlock nut end lengths , some ...