Is it illegal to smoke in a car?

In the UK, it is illegal to smoke in vehicles with someone under 18 present. The driver and passenger could be fined £50.

Source: Secondhand smoke @ NHS Smokefree

Image by Challiyil Eswaramangalath Vipin from Chalakudy, India (cigarette.. like a cigarette in the rain) [CC-BY-SA-2.0], via Wikimedia Commons