They're completely different metrics. If you want to compare median income:
UK - $49,500 (https://www.ons.gov.uk/peoplepopulationandcommunity/personal...)
US - $83,750 (https://fred.stlouisfed.org/series/MEHOINUSA672N)
I'm struggling to understand what you're trying to prove other than that you don't understand statistics.
Point is the median person earns about the same as the mean disposable income. So the mean disposable income isn’t telling us too much about the quality of life, or financial state, of a typical American. From what I’m seeing, it’s just reflective of our increasing income disparity.
Taken to an absurd degree… the mean of my income and Jeff Bezos’ income tells is nothing about my income beyond “much less than Jeff”.
Okay, well the ratio of mean disposable income for the US to the UK is 1.59:1, while the ratio of median income is 1.69:1
So your preferred metric for quality of life actually shows a greater disparity between the two countries..