76mm F, 112mm R on a Brompton. Why 112mm (rather than 110mm, which is what short-axle SA hubs are normally set to at the factory) I don't know, but Brompton rear ends are strange. You space the hub to 112mm and dish the wheel as normal so it's central between the locknuts. Put it in the frame and it looks wrong, with the tyre a hair's breadth from the driveside chainstay but half an inch from the LH chainstay. But that's right, because the stays are asymmetric and it will be centred on the main frame, which is what matters.
Back on topic, there's usually old Raleigh Twenty stuff on eBay, but an old 3-piece SA hub isn't great quality and the Ambrosio would be a better bet.