I appreciate you making this post, and agree with many of your points. One thing I'd add is that the situation of strategic nuclear warfare is unprecedented and would be extremely chaotic. In my mind this significantly raises the value of more traditional (in the sense of having worked fairly well across human history) hedges against uncertain situations. For example, while travelling to New Zealand or South America might be a good hedge against worst case scenarios of a full strategic exchange, being near friends + family or being somewhere you are a citizen might be much better than being in New Zealand in the case of massive civil unrest or less-than-expected weather disruption.
This is a fantastic forecast, thank you. I understand why this would be beyond the scope of this forecast, and maybe it's simply too chaotic to forecast reasonably, but I think from the perspective of making a personal decision what I wish there were here is a forecast on the likely regional outcome of a nuclear weapon detonating on London (or any other major city). Something like a forecast, conditional of a nuclear weapon detonating on London, that an escaped Londoner stays alive >1 year somewhere not directly hit in the UK vs >1 year in New Zealand, given baseline but minimal disaster prep.
Mostly I just bring this up because the cost of leaving the direct blast/fallout zone of a major city by travelling an hour or two is significantly lower than the cost of say, moving a family back and forth from New Zealand each time there is an escalatory step taken. If the only reasonable escape option is a significant trip to the Southern Hemisphere then it changes the cost side of the calculus pretty drastically for a personal decision (vs. just getting out of the immediate danger radius).