Are you paying less, all in? (e.g., taxes)
And I think if you have private insurance (i.e: double paying), you'll probably be paying more than the average American. I don't know for sure though.
On top of that UK private insurance is much cheaper because it is 'top up' - you're usually expected to see NHS GP's first etc. and use NHS emergency rooms.
As such while it is possible to end up paying more if you're very high income, very few people in the UK will pay more even with private insurance.
You also have to factor in the (very large) public expenditure on healthcare in the USA. Americans also pay for part of their healthcare through taxes.