FWIW Patterson Metro uses a 28T cog, and the same 1.6X overdrive makes a 45t
It is cable shifted from the handlebar, No experience on how you would adapt that to a fully housed cable
as required by the Brommy.. Your LBS may help Fabricate a technique,

also IDK what the crank chainline is..

Other people like Steve Parry in UK have clamp on FD mounts for Bromptons
to run 2 chainrings..

There is, of course, the simple heel Kick down, finger up-shifting by hand, with out a FD.