I have dealt with Classic Automobile Service and they are excellent! They have a new 15/6 Radiator for 741 Euro + VAT. The link is https://www.cas-shop.com/en/product-categorie/cooling-system-grill-heating-15cv/. I do not know the differences between a Big 6 and a 15/6 radiator and the experts may not like this idea. But I have a Big 6 with a 15/6 radiator and it looks and seems fine!
As a point I guess you can buy this less VAT and then pay the UK VAT when it comes into the UK. Needs some thinking about but has to be worth it!
Julian Pratt