Personally, I would suggest that in the cases of
both United Earth and the United Federation of Planets, it might be helpful to think about the United States of America. (My knowledge of the United Kingdom being much less, not being from there.) Yes, there is an overall sovereign state with a supreme government, legislature, judicial system, military, embassies, etc., that encompasses the whole and speaks with a single voice to the rest of the world, yet within it are many smaller states which, while not
fully sovereign unto themselves, nevertheless retain their own individual governments with
some measure of autonomy, the capacity to make and enforce their own laws and regulate their own activities, with their own agencies and police forces and courts to effect this, to the extent that such does not directly contravene federal law. Certain powers are reserved to the federal government, and others are reserved to the state governments. Coordination and cooperation between federal and state governments and agencies is fairly routine, whether as a matter of strict requirement or simply agreed practice.
Thus, it is entirely
possible that what
were thereto fully sovereign nation-states such as Australia
continue to exist in
some form (perhaps diminished, but still distinct) under the United Earth World Government, just as the governments and agencies of Earth and Vulcan (which even seems to still have its own embassy, per TMP) and Ardana continue to persist within the UFP.
(As for the Royal Navy, that
could well have been incorporated into the United Earth Military, but retaining its historical name as a unit within that structure. Or it
could have been entirely disbanded. The former could have happened well
before Malcolm made his decision to join Starfleet, and the latter decades
after. Starfleet has apparently been around since the mid-2130s or so, at which time Malcolm would have been in his late teens.)
Then why the hell did you bring it up?
Simply to point out that some of the things being assumed in any of our theoretical scenarios haven't in fact been directly spelled out and nailed down on screen, I presume. What is the point of having the discussion, if not to discuss any and all of the various
possibilities in play?
-
MMoM