Wow, retired in London. I don't think I'll ever afford to retire just living in the exurbs of the Midwest US. Kids and a spouse can push out the retirement goal considerably. Then there are medical issues and the extended family you'd hate to see on the street ...
If you exclude mortgage payments or rent then living in London is probably cheaper than living in most places in the US. We don’t have significant real estate taxes, my annual expenses on housing exc rent or mortgage payments (energy, service charge, internet, water, council taxes) is around 7-8k$. If you want health insurance it’s another 200-300$ a month but we have the nhs (albeit it is terrible for anything non-life threatening).