Obviously something has gone wrong and the Ebay gremlins think Wales is not part of the UK any more!
See if this workaround helps - add the specific buyer to your exemptions list and see if they can then make purchases from you. They should check that Ebay are not going to add VAT/import taxes if Wales is now overseas!
Allowing buyer exemptions
To exempt particular buyers from your requirements, you can add the username to your Buyer requirements Exemption List.
Update buyer exemption list - opens in new window or tab
"There are two ways to be fooled. One is to believe what isn't true; the other is to refuse to believe what is true.”
Søren Kierkegaard, Danish philosopher (1813 - 1855)