It could be argued that "Britannia" means the Welsh as much as, or more than, the English.

The name "Britannia" was the name used by the Romans for this island. In their time most of the people in Britain spoke early varieties of what eventually became Welsh. (As the Roman Empire fizzled out so did the Latin language for everyday purposes in Britain, whereas decendants of Latin are still the languages of the parts of mainland Europe that were under the Roman Empire.) Insofar as there may have been an Arthur, he was a Briton (i.e. more or less a Welshman) fighting against the Saxons (i.e. more or less the English).

"Rule Britannia" treats Britannia as a single entity. It does of course reflect the days of Empire (and, I suppose, also now the days of Brexit). But does it also reflect the dominance of England?