The reason for the pay discrepancy is less that the UK is an outlier, and more that the US is an outlier. Salaries in the US for software engineers are the highest anywhere in the world: salaries in the UK are very good nowadays relative to most countries, but the US is leagues ahead of everyone.
Could be, but I don't think it's health care. Check out the OECD stats:
https://data.oecd.org/healthres/health-spending.htm
The US in total pays $12318 dollars per capita in health care. In the UK, it's $5387 per per capita. The US's supposedly more efficient market-based system turns out to be wildly more expensive than any other country.
Not saying this is good or bad. Just curious about the reason for the pay discrepancy.