It seems the Cateye volt 300 is the new official Brompton battery front lamp (see on Brompton Bike builder page):
it has a fork/brake mounting bracket so as to remain in place when folded, although I would also fit a handlebar bracket for when using the lamp (and use the fork one for storage....)
I think other cateye lamps might fit*: so perhaps one of their German approved 2AA cell models would allow full law-abiding-ness for the keen
...
*assuming fork bracket sold separately.