Given that we’re obliged to contribute to a universal health provider, there is a legitimate public interest in criticising avoidable behaviour which increases the burden upon it, whether that be over-eating, lack of exercise, or substance abuse.
If the time comes when it is possible to opt out of contributing to that system (rather than merely consuming it), and choosing one that rejects the wilfully unhealthy, that legitimate public interest will no longer exist.
Now, please look at this more closely. The junk science the UK citizens are fed gives them even more reason to legislate thinness. Why, oh why, does anyone believe junk science would magically “go away” with universal healthcare? You’re still going to get outsourced groups writing the medical guidelines, and they’re still going to claim that fat raises risks in all cases, and they’re still going to recommend that fat people need to be eradicated.
Excuse me, but I’m already hated enough for my body in the USA. I don’t need a “legitimate public interest” in it, too.