It is illegal to smoke on an aircraft, call the police out, shows these people that it shouldn't be done. Ok having a word with them might work, but then others will see that they might be able to get away with having a quick fag and just get a ticking off from the crew!
The chances of the plane catching fire by a fag........... Slim ( but no less dangerous)
I'm a smoker myself, I know what it's like in the UK, 3 prior to the flight leaving there is know where to smoke, so I sympathise.