"Insurance" and "healthcare" are two completely different things. To understand why this is, observe that the military run their own state-run medical system rather than requiring individual soldiers to have private medical insurance for any battlefield injuries.
Non-US manage costs by some combination of state price control, aggressive triage, reduction of bureaucracy compared to complex insurance systems, and lower labour costs.
Also, in re "other countries are poorer than the US": that may be due to the US having a few extremely rich people, it may be possible that the average or modal American is poorer than the average European. Especially if you account for healthcare.
The one which has "US Tax rate 20.3%" and "UK Tax rate 18.9%"?
Even if we did pay more tax, how are you accounting for healthcare and other benefits? US state taxes? UK council tax, and the situations where you can have "negative income tax" in the UK (WFTC)?
And, as discussed, you're free of worries about whether or not you'll get healthcare in the UK. (You may experience a delay for non-urgent conditions, I'll give you that)
Non-US manage costs by some combination of state price control, aggressive triage, reduction of bureaucracy compared to complex insurance systems, and lower labour costs.
Also, in re "other countries are poorer than the US": that may be due to the US having a few extremely rich people, it may be possible that the average or modal American is poorer than the average European. Especially if you account for healthcare.