Yeah, that kind of seems like a dick move to whoever owns them. If something truly has that kind of value to a country, I really think that a country should be prepared to purchase it.
Besides, unless it’s in some kind of museum or something – which the government buying it could ensure – then it’s not as if anyone’s deriving a whole lot of benefit from it. Is there truly any benefit to the country at large if Artifact X is in a private collection…just a British collection?