We already pay
The other issue is that there would have to be all sorts of exemptions such as children, the elderly, pregnancy, people on benefits and people on low incomes.
That leaves a fairly small number who would pay and it would feel unfair as the above catergories are probably the bigger users of the health service
It would then breed more resentment about how the NHS is funded, giving political opportunity to do away with it even more.
Cynical.