> I'd argue that someone working for Google in the UK should also be paid the same.
Tax rates are very different in the UK and SF or Chicago. Level of social services provided to people is very different, nearly everything is different. I am not saying whether person should or should not be paid the same, but rather pointing out the fact that things are different between UK and SF/Chicago. Most of Google Engineers in the UK pay 40-45% income tax https://www.gov.uk/income-tax-rates
Whether you work for Google from SF or Chicago makes no difference to the company as long as you are doing the same work. I'd argue that someone working for Google in the UK should also be paid the same.