The vintage Raleigh stems have that same small diameter clamp, and are shorter than the original stem. Actually what I was suggesting was taking the stem from a 70s or 80s road bike (or new if it is available in a bike shop), 50-60mm stem length and use that.

The current suspicions that we have about its origins indicate that it originally came with drop handlebars combined with a short stem, a setup displayed by photogravity's bike. Another possibility which might be forced on you due to the availability of quill stems in BA, is using the combination of a longish stem, combined with a Porteur bar as demonstrated by nlerner's bike.