Just to be different, when I replaced my Brompton-issue Lyt front light (which rubbishly lacks a standlight) with a Cyo, I replaced the run to the rear light with some of that lovely Schmidt coaxial cable, simply because it looks better (being thin and black, rather than dark grey and floppy). In my defence, I was already committed to re-cabling the brakes and gears from scratch due to a non-standard handlebar configuration, as well as incorporating the cable for a wired computer, so it didn't add much to the required level of messing about to find the right lengths (which was exactly as tedious as it sounds).
So far it seems to be standing up to wear just fine, so it seems like a reasonable alternative. I'm not sure that the standard figure-of-8 bell wire that B&M supply would last long, given the amount of flexing and rubbing it's subjected to in the bottom bracket area during the fold, so it's worth avoiding that.
On the gripping hand, you still need Brompton's version of the front light bracket - the regular one won't clear the brake and the luggage block.