In theory, if you own and live in a £1m home in London you are very wealthy, you could sell it and live off the proceeds in a cheaper place. But in practice you are not really wealthier than someone who owns a £200k home in a village. That home is likely much smaller and the population of London is not going to migrate away from London. Real estate bubbles have made parts of the population much wealthier on paper but they still live in the same houses than before the bubble.
Actually income has a similar bias. You can earn twice the median salary in the UK and still not afford to live on your own in central london. Most junior bankers live in flatshare, but are on paper earning much more than an average family in a cheaper village.
These bias are what I think make comparisons of nominal wealth or income between countries a pointless exercise, and in a country as vast and heterogenous as the US a difficult exercise.