"Britanniarum Omnium Rex" is plural: "King of all the Britains" - meaning all of the British Isles, and anywhere else that "British people" might be found. The "Britanniarum Omnium" part was finally dropped shortly after Queen Elizabeth II took the throne, in recognition of the facts that (a) there was now a large part of "the Britains" - i.e. the Republic of Ireland - that no longer recognized the British monarch, and (b) that the other Commonwealth Realms - Australia, Canada, New Zealand, South Africa etc. - were now independent countries in their own right and no longer thought of themselves as "British colonists".
Don't say "infinitely" when you mean "very"; otherwise, you'll have no word left when you want to talk about something really infinite. - C. S. Lewis