According to Gallup in 2013 the median incomes in the US and UK were $15,500 and $12,400 adjusted. Considering that US citizens pay on average 10% of earnings on healthcare where the UK doesn't that makes it more akin to $14,000 vs $12,400.
Except here is something you didn't know - that British citizen isn't paying income tax at all. Their 0% bracket goes all the way to about $16,000 USD Today. That US citizen is paying about $1650 in income tax because there is no 0% bracket in the US. So in actual take home income they are almost exactly the same. The mess of tax code in both countries makes the calculations more complicated, plus other taxes, etc - but the point is that no, U.K. citizens are not being paid "so much lower" than the US on average, but they do get the benefit of never dying of preventable illness because they can't afford to see a doctor while having all that extra aforementioned paid time off and holiday benefits that US workers don't have.
Additionally, the US isn't even at the top of median individual income. Fennoscandia collectively has the US beat, by on average about 20%. Which is going to be offset by their average ~25%ish income tax, but still its comparable returns.
It turns out that if your economy is abusing you to get more productivity out of you that you don't get a better living out of it.