Do the fenders inhibit folding much?
Brompton's own? No.
Should I get one of those Brompton Bike Bags?
Their front bag mount is Great.
the S bar being Lower, interferes with the height
of the top of all but the lower S bag.
Because the brake levers have to swing above the bag,
on the frame mounted clip.
so need the clearance.
the many more bag options are designed around the other 2, M and P
bar /steerer tube riser options..