As this is a relatively new set up there isn't the wealth of knowledge out there yet about what can and can't be done to improve the gear range so everyone who experiments is a test pilot for those who follow.
It does seem, though, that the bigger chainring with a smaller inner as several have suggested is the way to go. Is the bottom bracket length significantly different on this model, and clearances on the rear frame? I've managed to do it on an older hub geared model, and that took a bit of trial and error, due to the concerns about any modifications not only working but potentially affecting the fold. Still using the greasy finger change at the moment but eventually might fit a front derailleur as I've got a frame bracket from
ebay knocking around somewhere.
With an unlimited budget you might go as far as changing the wheels and managing an extra rear cog but it'll be non standard and might need other, non reversible, modifications to make it work. In my case, I try to use standard parts as much as possible with a view to reverting to original spec if I come to sell it for example (though if I did sell, it would probably be to buy a newer, better specced Brompton)! At the moment, this one suits me fine, and I have the pleasure of knowing that any improvements are all my own work.
