In order to protect children and young people from the dangers of second hand smoke, it's illegal to smoke in a car or other vehicle if anyone under the age of 18 is present. The legislation applies to both England and Wales.
If someone is caught smoking in a vehicle with children/young people under the age of 18, both the driver and the smoker could be fined. The fixed penalty notice fine for this offence is £50 (for person who smokes and for driver) - someone who committed both offences could get two fines. The law does not apply if the driver is 17 and is on their own in the vehicle. However, if a 17 year old were learning to drive and their supervisor or anyone else in the car was smoking, the smoker would commit an offence.
The legislation applies to all private vehicles that are wholly or partly enclosed by a roof even if the windows or sunroof are open, the air conditioning is on or the person/s who are smoking are sat in the open doorway of the vehicle. However, the law won't apply to a convertible car when the roof is fully down or work vehicles/public transport (it is already against the law to smoke in such vehicles).
The legislation doesn't apply to motorhomes or campervans (motor caravans) and caravans when they are being used as a home. The reason for this is that the legislation is intended to apply to vehicles not homes. The legislation doesn't apply to e-cigarettes (vaping).
If you have a vehicle that allows you to remove roof panels but does not allow you to remove supporting sections, it will ultimately be a matter for a court to decide whether the remaining sections constitute a roof for the purposes of the legislation.
