Brompton isn't really compatible with a derailleur
6 speed Bromptons , now have a 2 speed derailleur/aKA Chain pusher.
They just do it differently.
2 pulleys with guide flanges molded in, move sideways. the shifter moves the pulley.
3 by 2 hubs have 2 sprockets on 2 different sourced hubs.
TW Sturmey now, Shimano bmx spline.
Previous Sachs /Sram uses 2 3/32 sturmey AW3 spline cogs
the 13 & 15 t cogs work to half step the 3 hub ratios, for the 6 gears.