The ideology of the nanny state can perhaps be summarised as coercing people to be healthy. Complaints about the first half of this (coercion) are well known: people should be able to make their own choices even if this involves them opting to smoke or drink or eat sugar. However, most people would regard the second aspect (health) as uncontroversial. Whilst the means of nudging, taxing, banning, and regulating may be objectionable, the end of promoting health is obviously a ‘good thing’.
I’m not so sure that this is true. Obviously, health has its benefits. Most people would prefer to not have typhoid. A healthier population may also be more efficient and happier. And, if you enjoy your life, it’s rational to want it to last and to die at 80 rather than at 40.
This is, though, only in the aggregate and on average. What about at the margins? It is less clear that there is necessarily any welfare gain in extending your life from 65 to 75 at the cost of personal pleasures like cigars, whiskey, and red meat.
And there are, perhaps a small minority of, individuals who would quite happily die at a very young age if this gave them significant immediate benefits. The same is true at a social level. Whilst, say, a population consisting exclusively of obese binge drinkers would probably struggle to function, it is not obvious that GDP (let alone net happiness or preference satisfaction) would be maximised by a country of puritanical workaholics. Likewise, whilst smoking or obesity may create extra costs for the health service, this is at least partly offset by shorter lifespans.
In isolation, this point is probably insufficient to prove that paternalistic public health policy is usually a bad idea. However, it should be taken in the context of governments ineptly attacking healthier substitutes to harmful products (e-cigarettes), the inevitable flip-flops from using incomplete data (is fat or sugar the enemy?), the inevitable ineffectiveness of policies (minimum alcohol pricing), the creation of black markets (drugs), and hysteria developing around things that have no proven negative health implications (GMOs).
Combined with the implications of a nanny state culture for liberty and personal responsibility, recognising that health is not an unqualified good suggests that policing lifestyle choices will always tend to be bad policy.
As an aside, I suspect that one of the reasons that the UK in particular has embraced health fetishism to such a degree is that, as an expression of our faith in the national religion of #OURNHS, we have elevated doctors to having the status of priests.