The Williams 1200 and 1232 are beautiful, but almost impossible to find online in a matching set of crank arms--I spent 6 months looking, then gave up. Even unmatched, they are extraordinarily rare. If you're going to search for something so rare, I think the Holdsworth Allez cranks would be more on theme with the bike. Alternatively you could go higher end with Chater Lea.

In my opinion the Durax Super Course cranks equally nice, easily available, and much less expensive. Don't know if they were ever available with the British spindle diameter.