Nebula

But it can overrule states in other areas, so in practice similar to the UK lording it over Wales or Scotland.

Westminster can overrule the very existence of Scotland and Wales as anything other than an idea in people's heads. Canberra cannot do that. It's not even remotely similar.

you seem to be confusing countries with nation-states

No. In fact, there's a better argument that Scotland, Wales, and England are separate nations than there is that they are countries. To quote a couple of helpful Reddit comments:

Nation has more to do with people than territory. It implies a group of people with a unitary identity based on language, ethnicity, or culture. Nations may or may not be sovereign (e.g. the Navajo Nation exists within the United States). A nation-state is a sovereign state that consists primarily of a single nation.

A nation is a unit of cultural organisation, defined by what the nation wants but often shared language, history, values, ethnicity, or a combination thereof (different nations are different).

A country is a nebulous concept often used to mean a nation-state, ie a state created to be the political side of a nation, an organisation in which the state and nation are congruent.

If they gained independence, the nations of Scotland, Wales, and England would then also be separate states, creating nation-states. As it is today, the UK operates under the idea that the "British" are a single unified nation (I've heard it called a "country of countries" before, but "nation of nations" would be much less of an obvious furphy), but nobody pretends they are actual states (in either the way France is a sovereign state, or the way Queensland is a federated state).

I also wonder what guidance the home game gives. Maybe someone here has a copy and can tell us.

I managed to find a rather poor-quality scan.

Administrative Divisions

1st Administrative Division

This is the biggest formal category of division. For the US, it would be states. In Switzerland, cantons, in Japan, prefectures.

2nd Administrative Division

[US -> counties, Switzerland -> districts, Japan -> subprefectures]

3rd Administrative Division

One more level down. In the US, Switzerland, and Japan, this would be municipality. Municipality borders can be occasionally difficult to define, so it's up to the seekers to clarify any ambiguity.

4th Administrative Division

Some places have no fourth administrative division, but many larger cities do. For example, New York City has boroughs. Zurich has districts. Tokyo has special wards.

Unfortunately, the advice in this section is less than helpful. It ignores different sizes of games. This is in the "matching questions" section, so questions like "are you in the same municipality as me?" I guess in the small or medium game you just ignore 1st and 2nd administrative division entirely.

In a London or Sydney–based game, I would probably use the city councils as the 3nd administrative division and suburbs for 4th, but maybe someone with more knowledge could address it better. (I'm not sure about "suburbs" for London. I'm aware that Australia uses the term very differently from what it means in America and Canada, but I'm not sure about the UK.) Here in Brisbane I can't city councils are probably too large (the City of Brisbane is approximately the same land area as Greater London, and Greater Brisbane, which includes 4 satellite cities, is supposedly 10x that area), so probably that electoral boundaries idea I had yesterday would be the best option.

Side note: they specifically describe Greater London or any "major city" as being their Medium game, with Small being "a single town, small city, or portion of a large city". If you wanted a small game, I suspect a single London or Sydney council would be too small, so you'd probably choose a handful of councils to play in, or base it around some local feature/cultural area.

There's also a "Borders" section within the "measuring questions" ("compared to me, are you closer to or further from...?"), which includes 1st and 2nd administrative division borders, as well as international borders. 3rd & 4th administrative division borders are not available for measuring questions. The same description of what these mean is listed here.

Bonus: I also saw answered a question that has bugged me for a while. How to do the Strava map, since Strava itself doesn't have a way (that I know of) to show the path without the streets.

this can be a little tricky to do; we have a few methods. One is to screenshot this on your phone, then use the drawing tools on your phone's photo editing app to black out everything but the street. Another is to put a piece of paper over your phone and trace with a pen/pencil/marker.

Not that it's very relevant. This question is only available in Large games anyway.