Nowhere in law is it stated that it is illegal to eat or smoke whilst driving. What is illegal is to drive without due care and attention and you might be doing this if eating or smoking, but then again you might not be. It depends on the way you are driving and whether it appears that you are not exercising proper control over your vehicle. It is hard to prove in court unless there is an accident.
Mmm smoking at the wheel. There you are having a tab when the end just happens to drop in your lap. The next thing you are doing is frantically trying to leap around in your seat trying to put it out. Now if that is not a distraction at the wheel while driving i don't know what is.
Oh and that was speaking from experience when i used to partake in the filthy habit. So having a tab while driving really does fall into the same category as using a mobile in my eyes. You could go onto drinking a hot beverage as well. Scalding coffee or tea spilt on the family jewels will make you leap around like a demented idiot.
My late father was a driving instructor for a while back in the 1960's and he once said smoking in the car had saved his life!......he was instructing a driver from the passenger seat when the learner driver froze at the wheel and the only way my dad could make him let go of the steering wheel to avoid a head-on collision with a Lorry was to stub his lighted cigarette into the learner drivers left hand. He immediately released his iron grip on the steering wheel and my dad was able to steer the car away from the impending collision!