You raise a possible valid point al but the engines are not too near the fuel tanks so I guess the pilot allowed people to grab their belongings as probably well organised by cabin crew. Pilot would probably know how much time they had to evacuate. The fire service was there very quickly too as they are parked up on the tarmac.

Many years ago I had a similar experience on a flight from Heathrow to Manchester where the cabin began to fill with smoke just as we were coming in to land at Manchester because one engine had caught fire. We could see the fire engines on the ground following us up the runway. When we landed they got 1st class off first, as Edward Heath was on the flight... luckily there weren't many of us in cattle class so we did exit down the stairs, not the shute. Cabin crew very calm and organised. I think probably the difference was we were in the air, this BA flight was on the ground so more time to grab luggage and evacuate. I my case I didn't have luggage as it was a day return business trip.