I have the same problem (here in the States) for my 1951 Plus 4.
Only expensive Radials. Bias Ply available at a significantly cheaper price. 100$ cheaper per tire. Internet searching reveals only Coker Tire.
I would go to the UK, and even pay shipping, if I could find a better solution.
I don't thrash the car so I guess I will have to go with Bias Ply. That's what is on there now and they have been ok, but . . .
Cheers,
Mark