The chainring is swaged onto the Crank arm, of the standard Stronglight made for Brompton cranksets.
So a change to a crank which bolts on a chainring will handle more torque,
say adding a second chainring for mountain climbing.

the swaging connection has come loose on the stock cranks,
like putting a 30t, 86bcd chainring

Hilly west coast,
I fitted a Schlumpf Mountain Drive crankset, 54t, 130 bolt circle.
the BSR has a 15t (3/32 wide from the Mk3, Sachs 2by3 hub)

Florian's company makes a Brompton Specific Model.

neither crankarm is connected directly , the right one turns faster
than the chainring , in low range.

the reduction gear torque strut has a knob that sits above
the BB /rear pivot portion of the frame.

the internal reduction gear has a big drop, so the next lower gear,
from the low gear in the hub, direct range, is low range, High Gear.

so a wide range 6 speed .. [pretty = %, 33, 41 at the double shift]
the planetary low is a 21.6't' ..

with a 13t cog a 47t chainring will do similar .. ~18" ~ 80" range..