I’ve recently had problems sourcing 20 inch tyres too. I emailed Coker last October and was informed that some of their 3.00 x 20 blocked-tread tyres would be put on the next shipment to their UK stockists (North Hants Tyres -I think) with whom they had a sole distribution agreement. The reply was quick and helpful and suggested that an alternative would be to order via a US address for onward shipping to the UK.
I put an order in for 4 and was originally quoted December delivery. In subsequent telephone calls HT have merely informed me that the tyres have not arived and that my order is on file. I haven’t been able to chase them recently.
As with anything a larger order is likely to cause more interest (which is partly why I was ordering 4 tyres, having joined my requirement with that of a friend). My mother-in-law is in the States soon and will be able to provide the US address that Coker suggested could be used. I am prepared to accept and coordinate orders to add to mine if anyone else is interested. I am not fully aware of the costs as yet ,but those interested may let me have details on a non-obligatory basis and I will report back.