I’m British, lived in the USA since 2005.
And I think the basic answer to your question is that the standard of living in the UK is more or less the same as the USA. There, just as here, of course, it depends on how wealthy one is.
Things are, in general, a bit more expensive there. There are exceptions, of course, and also folks tend to get slightly higher salaries for the same jobs, so that evens out.
Houses are often smaller than in the USA, and since the country is a lot more urbanised, many more people live in city apartments or condos (the word “condo” is not used in the UK).
My parents run two cars, and while they’re certainly not broke, they aren’t outrageously rich either. So I wouldn’t raise an eyebrow if a British person told me their household had two cars - it’s reasonably common.
Welfare payments there are more generous and cover more people, so there are far fewer people in crushing poverty than in the USA. Of course, taxes are higher as well.
London is different - living there is ridiculously expensive. The six years I spent in the city, I live in dilapidated apartments just so I could pay the rent. Many people who work there live in suburban “dormitory towns”, and so spend a huge amount of money on their train commute. Lots of London employers offer interest-free season ticket loans to staff - my commute from Rochester to the City (London’s financial district) ran about US$5000 per year. The expense of living in London can be a real problem for modestly paid workers, like teachers, nurses, police officers and firefighters.
So the basic answer is that, if you were to move to the UK tomorrow and have a reasonable job, you wouldn’t likely notice much change in your material standard of living. However, you would have to get used to a smaller house and denser population in most areas.