> everywhere in Britain that's outside of London is now poorer than any given state in the US.

This is true of Europe in general vs the US because economic growth in Europe has been low compared to the US since at least the financial crisis. At the time GDP of the Eurozone was comparable to the US' now the US is almost twice as big. As a consequence every European country ranks low compared to US states on GDP per capita:

"Italy is just ahead of Mississippi, the poorest of the 50 states, while France is between Idaho and Arkansas, respectively 48th and 49th. Germany doesn't save face: It lies between Oklahoma and Maine (38th and 39th)." (2023) [1]

[1] https://www.lemonde.fr/en/opinion/article/2023/09/04/the-gdp...