The question is will France stand for arresting people for smoking in Paris if they travel to the UK. The gonernment of France can allow that or they can retaliate in various ways. Just a diplomatic message is likely enough to make the UK back down - but who knows maybe the two won't agree and go to war.
Not really. It's more like DPRK messaging a private US citizen directly, repeatedly and incessantly, that they will be executed for blasphemy. Ofcom is not using proper diplomatic channels here.
Why should parisians care? Why would France cooperate with enforcing such laws?
Everyone here seems convinced that Parisians should care about this, because the majority opinion seems to be that it's perfectly acceptable for the UK government to arrest Parisians for having ever smoked a cigarette in Paris, should they set foot on UK soil. I do not agree that this is a defensible application of law.