The website that you're citing is a British site for the British monarchy.  It is a government site maintained by the British monarchy.  The website for the Canadian monarchy is run by the Canadian government and names Elizabeth II as the Queen of Canada, as does the website for the Canadian Governor General, the Queen's representative in Canada.  The website for the Governor General of New Zealand refers to her as the Queen of New Zealand.  This site is run by the government of New Zealand on behalf of the Queen's representative in New Zealand.  And so on.  Saying that because the official website for the British monarchy refers to the Queen as the Queen of the UK and the other Commonwealth Realms it's appropriate for Wikipedia to do so is displaying a bias towards the British monarchy over the other 15 realms that also have the Queen as the head of state.  Furthermore, on the website for the British monarchy the Queen is referred to as "Queen of X" when you go into the individual pages for her realms, where X is that realm, not "the UK and whatever realm's page we're on."  In fact, in all of the pages it says some variation of "The Queen's relationship to St Vincent and the Grenadines is unique. In all her duties, she speaks and acts as Queen of St Vincent and the Grenadines, and not as Queen of the United Kingdom."