If Britain were a US state it would be between Mississippi and Alabama in GDP per capita.
Not sure I’d call the place well off. Granted, a lot of the reason they’re in this situation is because they put everything into stopping the Nazis and thus saving the world. The US kinda lucked into inheriting the UK’s wealth.
GDP is not a measure of wealth let alone well being, it just means Mississippi is more expensive than Cornwall.
Japan is even lower on the list, but personally I’d rather live in Osaka than Mobile.
Though with (for the moment) a rather better social safety net / services provision than the US as a whole, and MS/AL specifically.
I do agree with your larger point, it is however counterbalanced in part. Gross per capita income doesn't reveal the full picture.