'From 1 October 2015 it will be illegal to smoke in a car (or other vehicles) with anyone under 18 present. The law is changing to protect children and young people from the dangers of secondhand smoke.
Both the driver and the smoker could be fined £50. The law applies to every driver in England and Wales, including those aged 17 and those with a provisional driving licence. The law does not apply if the driver is 17 years old and is on their own in the car.'
https://www.gov.uk/government/news/smoking-in-vehicles