Some stagnation is to be expected from high energy prices and trade disruption (brexit).
British surveillance state tolerance has always been pretty high for Europe, and is typically "sold" to the average citizen as anti-crime.
Almost all the wealth is in the South East of England. Outside of that the country is much poorer.
I drive from Manchester to Dorset once a month to visit my parents. There is a clear line where I notice all the street signs, the service stations, roads etc are better kept. Cars and houses are in better condition/news.
Purchasing power adjustment is ~1.5 (in favor of China, obviously), so this should not be close no matter how you slice it.
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.