Threats to deny NHS care to those who don't maintain a healthy lifestyle are a paper tiger. We're not going to see smokers, the overweight and drunkards dying in the street with Gordon Brown standing over them wagging his finger and saying "look at the NHS Constitution". Even if the politicians were hard headed enough to do it, which they aren't, they'd struggle to bring the medical profession along with them.
The only thing that might actually happen is the denial of certain forms of treatment - particularly expensive treatments. Even that probably can't be institutionalised as their are too many borderline cases. Smoking aside, most lifestyles will contribute to health risks instead of obviously causing a condition and in uncertain cases it will be hard to deny treatment. Consider the pretty common case of a man who is somewhat overweight but not morbidly obese and also has a stressful job. We wouldn't want to punish someone for having a stressful job so it would be a moral nightmare to deny that man treatment. Cases like that will be very, very common.
That means decisions over whether to deny treatment will have to be, from the point of view of the public, highly arbitrary. The threat of denying NHS treatment is too idle, too vague and too uncertain to have much of an effect on incentives. It can't get around the problem that a taxpayer funded healthcare system makes each person's lifestyle choices a public good or bad.
Now, every healthcare system that pools risk faces this problem to a certain extent. If you have a compulsory social insurance model and mandate that all insurance policies are available to everyone (as the Dutch do, for example) so that no one is going to be left without treatment and the vulnerable (thanks to age or existing conditions) are looked after you might also wind up subsidising unhealthy lifestyles. However, there is a key difference.
When individuals, rather than taxpayers, are paying for their health insurance it should be possible to allow adjustments in their premiums for healthy behaviours. As Wat Tyler describes this is already happening in the private health insurance market. It doesn't face any of the problems that I've mentioned above and prevents unhealthy behaviours imposing a cost on the health conscious.
Sin taxes are a clumsy substitute that have to make do without price signals (the level of tax is arbitrary whereas the correct level for lifestyle compensation in an insurance fund can be discovered by a market process). They are an easy way to raise revenue rather than a serious attempt to internalise the externalities associated with poor health. They will always be inferior to a solution that stops unhealthy lifestyles being an externality.
Pooling the costs of bad health and turning lifestyle into a public good is a problem for the NHS that leaks into the rest of public policy, turning it in a distinctly illiberal direction. There are few individual choices that don't have some kind of impact on a person's health. Unhealthy behaviour is controlled in an ever more draconian manner through taxes, regulation and outright bans. The Millian sphere (the set of decisions that don't affect others and should be left to individuals) is always unstable but completely dissapears with socialised healthcare. Liberalism becomes almost entirely untenable.
One quick aside: If the Liberal Democrats were an intellectually serious bunch they'd take this issue very seriously. The contradiction between healthcare socialism and lifestyle liberalism is one more example of the kind of issue that makes an attempt to marry their attachment to Britain's socialist institutions with a notional commitment to 'liberalism' a complete farce.
Liberalism isn't just undermined by the real externalities to unhealthy behaviour, created by socialised healthcare, that I have discussed so far. It is also politically expedient to exagerrate the problems associated with unhealthy lifestyles. The NHS, as it is currently organised, just can't deliver the performance that people expect and that would warrant the huge amounts of taxpayers' money that politicians have spent. That is a political nightmare for any political party that can't countenance calling for serious reform. They can dilute perceived responsibility for that institutional failure by announcing measures relating to people's lifestyle.
That is how we should understand this new threat to deny care to people living unhealthy lifestyles. It isn't even a serious but vain attempt to address the problem that the NHS makes everyone's health a public good. Instead, it is another attempt to avoid criticism for the ongoing failure to deliver quality healthcare and value for money in the NHS.