Liverpool is, according to reports, the lung cancer capital of Britain. Not a nice statistics I am sure. But simply slapping an 18 certificate on any film depicting smoking is not going to sort that statistic out.
Liverpool City Council are supporting moves by the group Smoke Free Liverpool to put an automatic 18 certificate on every film featuring images of smoking, even retrospectively, meaning that any film used in schools to support English literature (the likes of "To kill a Mockingbird", for example), would also be banned from schools.
Modern films set in a contemporary setting will, of course, have virtually no smoking in them because in the US and in the UK and most of Europe now, smoking in bars, restaurants, etc, is banned, so Liverpool should have no need to ban these. But the threat to go back and effectively ban hundreds of classic films from being used to support teaching in schools seems crazy.