ULINE is where I get mine up to 24"x36" here in the States. Since they come flat and are therefore very large, they have to ship via freight at that size. A 16"x20" would probably ship flat as 32"x 24"(Flaps on the end which must fold over as well). You might be able to have ULINE ship that size via DHL or Fedex but the shipping would be very high no doubt. Even domestically, the actual cost is about doubled when shipping is considered. Then you would have to be concerned with damage during their trip as they are just cardboard. Surely(and I know your name is not Surely), there must be a Packaging supplier in the UK with something very similar.