I imagine that it is the same universe as Children of Men. I heard the same theory, the rest of the world is normal, Britain just decided to be that way and become even more of a dysfunctional shithole.
The tragedy faces New Zealand is that as soon as Australia is rendered uninhabitable by climate change, millions of Aussies who previously advocated for concentration camps for "boat people" will migrate over and set up shop like nothing ever happened.
Thought we were gearing up for that during the COVID lockdowns, New Zealand looks like the bunker nation for when the billionaires' home countries collapse.
this is my assumption for basically all post-apocalypse and/or dystopia fiction. Unless you specify what's going on the rest of the world, I'm just gonna assume everywhere else is fine.