Hate to say it but the commentators above are correct. “The matter of Britain” dates to when Brythonic languages were spoken in the islands, as well as Brittany (of course). Usage since then in the ‘different parts’ has just become more confusing. Is Wales part of England? Is Scotland? Denizens of either region would emphatically assert otherwise, but poor Mr Trump is quite correct. For example, soldiers from those parts are still galled to be called part of the English army.
