What you cannot do is mix private and NHS care for the same illness (so called topping up).

This, I believe will change, in the fullness of time

You are probably right, but it is a bit of a double-edged sword. Once we allow 'topping-up' it gives politicians a loophole not to increase the allocation to the NHS each year as much as they otherwise would have to. Over time, I think that could lead to a withering of the NHS. Without top-ups, the service really has to work properly as funded; with it it only has to work if 5% of the funds come from elsewhere. Then a few years later if 10% are from elsewhere ...