It's a daft idea - where do you draw the line if someone injures themselves whilst cycling? Climbing? Doing DIY? These are peoples lifestyle choices as well, and it makes more sense to do what they do currently, which is to tax cigarettes, cakes etc. at the point of sale.
I'd also assume that obese people will on average have a much lower lifespan, so I guess that's a money saving mechanism built in.
I think it would be fair to draw a line between someone who is taking part in an activity that has a low potential for causing injury (that requires professional medical attention), but otherwise improves their health, and someone who is knowingly damaging their health through poor lifestyle choices.
Whats the point of the NHS if it only treats healthy people then???
How about for treating people who are ill/injured through misfortune, not because they deliberately drink/eat/inhale poison.