there is no comparison with a large national plan which guarantees coverage and insurance companies in the us vying for your dollar. the ones in the us will fight to deny your coverage when the time comes. the national plan cannot do that. that's the main thing you're missing.
you've created a straw man argument.
You're right, the national health absolutely will not, and can not, deny you coverage.
And that's why you have private insurance companies to fill in the gaps for situations where the national healthcare service won't cover you.