There’s another way to set this up that could get you want you want. Say, for whatever reason, D-Day failed. Germany, without the pressure of the western front, has the time and resources to progress on their own atomic program. A V2 delivers a nuclear payload to London, then the US drops the bomb on Berlin. War winds down leaving much of Europe an irradiated mess, and hello apocalypse
A lot of people head to the Americas or Africa, anywhere they can find pasture that doesn’t glow in the dark. But a lot of people can’t or won’t leave.