bar mount on the bottom of the drop suits me pretty well, still experimenting though
Brompton do a handlebar fitting extension for their handlebars
Depending on the size of your forks, this can be fitted, and allow ordinary battery lights to be fitted.
Beware of two things though, unless tightly fitted with a non slip material between fork and bracket, then there is a real danger of vibration movement, and as forks are tapered, this loosening can let the light and extension rotate into the spokes. Another answer is to fit above the brake bosses (if fitted) preventing this.