What about preexisting conditions? Would anything prevent someone who was just diagnosed with cancer (but not bed ridden) from migrating to the UK, paying this $300/year, and receiving full coverage?
They call this "health tourism" and its a fairly regular subject that comes up whenever NHS budgets are discussed, in reality though I believe latest figures put it at about £300m /year (0.3% of the NHS budget).
I don't know how easy it is to do what you describe but some people evidently do it. I imagine if you're an EU citizen you can come right in but if you're non EU you might have some more difficulty.
It doesn't cover tourists anymore in theory. They'd treat and then bill you but you could probably go home and not pay.