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.
The "Raw Lacquer" finish you already have gives you the raw steel and brass brazing filler appearance without exposing the bare steel to the elements. Unless you want your Brompton to look like an abused rust bucket, leave it the way it is.