Sheldon-Brown has an article on square taper BBs. I have myself used an old Stronglight ISO spindle with a Shimano Biopace JIS crankset. It worked fine for years. Not ideal but possible. It was not a Brompton and I do not know if the required chain line on a Brompton would be a problem. Sheldon-Brown, in the last two lines of the square taper BB article states:
I generally avoid mixing sizes on customers' bikes, but I have a lot of experience mixing ISO/J.I.S. in both directions on my own personal bikes, and it has never given me a lick of trouble.
My experience with the Brompton is that it is a bit tight or demanding with regard to the chain line. Especially when using a brand new chain.
If you can mount the spindle on the crank arm without bottoming out the square taper, then I think you will be OK. Not taking the chain line in
to account.