Below is a description of the event posted on youtube alongside some footage of the evacuation.
I've highlighted a section of interest to those that seem to think shouting "fire" though a cabin load of jittery pax is an effective option....
The video being being utilized was captured by me on the tarmac. The odor of the smoke quickly took your breathe away and burned the lungs. The odor quickly become more potent and the instant feeling of being a trapped animal rushed over me. The captain came over the PA and announced we would be heading back to the gate due to this intense odor. Moments later, directly across from where I was standing, passengers began to scream and yell, "FIRE!" At this point, what appeared to be chaos broke out in the cabin and a passenger rushed the emergency exit door and opened it while a flight attendant yelled not to. The passenger exited and at that instant the captain announced that we were to go to manual exit and the flight attendants quickly began opening the exit doors to get the passengers out as quickly as possible. The BA staff did a great job to get everyone out of the plane and onto the tarmac.