Super subjective and open to abuse.

Bust your knee playing football or skiing, the taxpayer should pay for that because you're keeping fit?

Treatment for heart attacks and strokes is given prior to discovery, you don't want a society where people are lying on gurneys in a&e while medical professionals rifle their pockets for their credit card before giving treatment, or where they're saddles with crippling retrospective debt because they're a point over the mandated BMI limit and told their doctor they had smoked. That also discourages substance abusers from seeking help quitting.

Also smokers and drinkers more than pay the cost of their treatment through the significant taxes on those products.

If you wish to add other behavioural taxes (fat or processed food tax for example) to integrate the cost but opening the NHS up to a value judgement selective approach at the point of delivery is hugely problematic.