My own Brompton's brakes are not the greatest either. For a temporary solution, I added the best brake pads that I could find (Kool Stops), rotated the brake levers a bit upwards toward my fingers (I have small hands) for more leverage without interfering with the fold, don't carry a great amount of heavy items, keep the brakes well adjusted, and don't ride in the rain or snow. When I am able to afford the upgrades, I will probably buy the Brompton dual pivot brakes in a couple of years when the brake pads are ready to be replaced.
I hope this helps.