Since 2016, it's illegal to smoke in a car with anyone under the age of 18.
Exposure to second-hand smoke in cars involves higher concentrations of health-threatening chemicals than in larger, open areas. Even if the windows are opened or air conditioning is used, harmful particles can remain in the atmosphere long after the visible smoke has disappeared.
Since 5 December 2016 it is illegal to smoke in a vehicle with anyone under the age of 18. You can be fined for doing for doing so. The maximum fine for this offence is £1,000, but you may be offered the option of a fixed penalty. The fixed penalty will be £100.
Opening a window does not get rid of the harmful chemicals from second-hand smoke. Therefore, no matter if you have the windows open and/or the air conditioning on, if you smoke in a vehicle with someone under the age of 18 you are breaking the law.