The official position is as rcspeirs says. However, the NHS is not that well set up to reclaim payment from foreign visitors who make it into the hospital system. But if you don't want to rely on their inability or incompetency at charging people for medical treatment, then you are right, you'll need additional insurance.

And frankly, although we're welcoming, given the state of our own finances, I'm not sure we should be offering free treatment to those who can afford to pay, expect to pay and will pay. But that's probably getting a bit political

Hopefully, you won't need it!