The Brompton bottom bracket is a special one, its asymmetric with the right axle 5mm longer than the left axle to compensate for the frame bottom bracket box asymmetry.

To my knowledge, there are only two possibilities for an asymmetric 118.5mm square taper JIS bottom bracket :
- either the Ti Parts Workshop one made specially for the Brompton (exists with steel bearings or ceramic bearings) it has a titanium axle and save about 100g vs. the original BB.
- either the Phil Wood BB that exists with titanium axle (similar weight saving as for the TPW) or steel axle.

If you want to further reduce the weight of your Brompton, you can also change the crankset, the original one is quite heavy and replace the chainring+cogs+chain with 11s ones that weight less (a KMX X11SL chain last much longer and weight significantly less than the original chain).

If you want to change everything, Ridea has a beautiful low weight crankset with integrated axle and external bearings specially made for the Brompton.