Well they done it. MPs have voted by huge margins to ban smoking from all public places including pubs and private members' clubs in England.
Let's open the book now: what are the odds on the first successful legal challenge?
My money's on a Rastafarian. While it's true the US ruled that Rastafari do not have the religious right to smoke ganja in violation of drug laws. And the Court of Appeal case of R. v. Taylor  1 Cr. App. R. 37, it was held that the UK's prohibition on cannabis use did not contravene the right to freedom of religion conferred under the European Convention of Human Rights and Fundamental Freedoms.
My guess is that prohibiting the social and cultural aspects of smoking is in contrevention. Surely people may gather and smoke a placebo...?