The rules of the smoking ban in line with British law were clearly “No smoking” in enclosed public places or substantially enclosed places and the London Heathrow airport is not an exception. There has always been a question of smoking areas among the passengers of London Heathrow airport whilst waiting at the departure lounge in the gap between the check in and the boarding.
The answer on a straight tangent is Heathrow is a smoke free airport. Perhaps, the smokers forced to be under such an unlikely scenario are permitted to smoke in the smoking zone outside the terminal building. However, You will end up spending a significant amount of time literally to get into the smoking area just to cage up that nicotine monster, You will have to go through customs, go to the smoking area and then pass through passport control to get back to the departure lounge. Smoking areas are found outside the entrances to all terminals. Also, the same being applicable to the connecting flight passengers.
In addition to the above, considering the passive smoking with the usage of Electronic cigarettes are still treated as regular smoking category and hence Heathrow applies the same restrictions of smoking, only in designated areas at the airport.