Bike in raw steel form would quickly acquire a "rust" brown/orange tint to it. While such patina might be desirable, steel oxidization is a destructive finish and would shorten the lifespan of your bike.
However, I bet Brompton might accommodate, and if not, you could always just strip the raw lacquer finish from your bike to achieve your desired finish characteristics...
Personally, I think a bike made of appropriate Corten steel tubing would be da ballz, yo.
I know next to nothing. I am frequently wrong.