You can't definitively conclude anything about example B from the results of example A if the two examples are different.
That's not what I'm saying though. All I'm asking is for you to show me how you will mitigate the problems that will crop up when you try to implement a similar system to the UKs at the scale of the US, because the US has 5x the population of the UK. That's not a negligible difference in scale, and if this can't be shown at all, that is a problem.
You assert that problems will crop up, but fail to mention which problems. You are arguing that any data anyone brings up is irrelevant, without bringing up any data.
You keep comparing the UK to the US, but a much more apt comparison is e.g. UK to CA, and the entire EU to the entire US. UK has twice the population of CA, and better bang/buck and health outcomes. EU compares similarly to the entire US.
The EU is at this point in time, a federation slightly more disjoint but ultimately of similar structure to the US.
Why not build an independent single payer healthcare system in each US state, with reciprocity among them? That is directly comparable to the european system, and has been shown to scale and work efficiently and well.
That's not what I'm saying though. All I'm asking is for you to show me how you will mitigate the problems that will crop up when you try to implement a similar system to the UKs at the scale of the US, because the US has 5x the population of the UK. That's not a negligible difference in scale, and if this can't be shown at all, that is a problem.