I've never heard of this, and can't find anything to back it up (which is why I asked). There's usually a couple of reported cases a year where people travel abroad for something that isn't (yet) approved or available in the UK, and others that are routine.

The closest thing I can think of is a cannabis medicine, which isn't available in the UK, but the children involved have been able to go to the Netherlands for it. The trouble was when their parents brought the medicine back.

So, I assume it's American anti-public-healthcare fake news.