Except it probably wouldn't save any money due to the administration costs of such a system and goes completely against the basic principles that make the NHS so valuable in the first place.
But as a smoker I apparently place an extra burden on the NHS, something I pay for through excessive taxes. It is my choice to smoke so I sort of have to weather paying the extra taxes and I completely understand. Someone coming to live here permanently and therefore enjoying the benefit of the nhs, yet not being able to communicate with the staff also place an extra burden on the nhs so why shouldn't they pay a tad extra for it, after all it was their choice.
As a comparison in Canada if you have children you pay a touch more into the pot, this covers the extra burden on the state of having to educate your children, which also seems pretty fair.