Europe

Two things.

1. Think of the colonial UK as an early model for the US. Yes, a small number of people got fantastically wealthy but they did so at the expense of the entire world, including most of the people in the UK. However, it was much more lopsided than the US. There was no boomer phase where everyone assumed it was their birth right to have a large house with a garden. Country poverty and city slums were normal. Colonialism has always been about sending your poor to die in wars, as part of oppressing other countries.
2. World war ii wiped out the UK finances. The empire collapsed, and it dropped from being a declining world super power to the "sick man of Europe".

Even before WWII, where do you think Marx lived when he was writing? For much of his life he was in London, and the poverty around him informed his writing.