Belgo, the highway code states:
^Safe driving and riding needs concentration.
Avoid distractions when driving or riding such as
loud music (this may mask other sounds)
trying to read maps
inserting a cassette or CD or tuning a radio
arguing with your passengers or other road users
eating and drinking
smoking^
But at present it is not a punishable offence to smoke on your own in your own car.
Work cars are a different matter, the HC states:
You MUST NOT smoke in public transport vehicles or in vehicles used for work purposes in certain prescribed circumstances. Separate regulations apply to England, Wales and Scotland.