I quite like the Brompton-branded Stronglight, I think it has a sophisticated sheen, but if you don't like it fair enough.

Be super careful if you replace just the bottom bracket as the standard bottom bracket unit has ISO tapers. You could get a Phil Wood with Titanium axle, Aluminium alloy cups and light easy rolling ceramic bearings...

Another point to bear in mind is that if you replace the crankset you loose the stub that sticks out from the backside of the left hand crank. This recent 'innovation' on Brompton's part means that the folding pedal can no longer hit the bike frame and scratch the paint.