Earlier quoted context omitted.
The UK doesn't put the country of origin on their postage stamps, because they invented them [1], so there were no ambiguities to lift. That's the same reason the US is +1 country code and holds .gov [1] https://en.wikipedia.org/wiki/Postage_stamps_and_postal_hist...
Also the same reason, I guess, English-speaking countries have 0 and 1 as country codes in ISBN numbers. https://en.wikipedia.org/wiki/ISBN & https://en.wikipedia.org/wiki/List_of_ISBN_registration_grou...
It is mentioned in the Wikipedia link, but buried a little. I realise ISBN history is the epitome of super-nerdy, but the evolution path from $just_some_retailer through to the Bookland "country" is really quite interesting from an interoperability perspective.