Why is everyone British?

I've managed to listen to the first few hours of Gideon the Ninth and I'm currently listening to Nona the Ninth, and I just questioning why no one is Kiwi?

We know that John is canonically from New Zealand, and he was the one who resurrected the people after the Earth died by nukes, so surely his language would have been the basis for the post-resurrection language? The people resurrected presumably wouldn't have remembered anything from before their deaths, so everything they learnt would have come from John, including their language.

I bring it up because, in the audiobooks, Harrow speaks with a noticeable posh British accent, and Palamedes and Camilla both sound vaguely British Northern. Pyrrha seems to be vaguely Russian (I'm not great with accents), Gideon has an Estuary/London accent. These could be excused for a variety of reasons; the circumstances of the resurrection aren't known to the extent that we know what accent people had post-resurrection, and it's been 10,000 years since then, and accents are known to drift over time.

But there's still one person who's accent we should be able to guess: John. And yet, in the John number chapters, and presumably in Harrow the Ninth as well, he doesn't speak with a New Zealand accent.

I like the audiobooks, but this seems like an odd choice. I get that it's rare for sci-fi and fantasy books to give explicit accents to their characters, especially when those characters are so far removed from Earth by both time and geography, but decisions were definitely made here. For instance, there's no textual evidence to suggest that the Sixth all have Northern accents, but they do in the audiobook. That makes sense as a way of differentiating the houses, but it's still an extra-textual choice.