I agree that this is probably a bad idea. Welding will be an awful lot of work--removing the old drop outs, welding, some significant repainting, and realigning the dropouts. I haven't spent much time with Bromptons but I'd also be concerned it'd compromise the fold. Have you considered getting a Sturmy Archer? They make a 120mm spaced hub:
https://www.treefortbikes.com/Sturme...F8-8-Speed-Hub . It looks like you may be able to narrow it a little by reducing the size of the spacer on the NDS, and then that should be in a sane range for cold setting.