e.g. Jobs in London (even remote ones) will pay twice as much as jobs in the North West of England.
From what I can tell the prosperity gap is also large enough that small errors don't really matter; I already looked at urban china vs average UK (systematically favoring China) and the numbers are still not close (something like $30k vs >$40k even after PP adjustment).
Using the Median as far as I am concerned is meaningless as the South/South-East skews everything. It is quite obvious as you drive across the country. You can see it with your own eyes.
I don't really care that much about the comparison with China.