In British low, smoking is prohibited in public areas. And it is relevant to the Heathrow airport too. 

• Smoking in the areas outside the terminal is allowed. 

• Smoking is not permitted when the travelers enter to the departure lounge through security control.

• Travelers are not permitted to use electronic cigarettes too.

• When carrying cigarettes in hand luggage, it is essential to check with security to ensure that those are allowed to carry. 

Designated Smoking area at Heathrow