The more I think of it, the more convinced I am that this request is to do with the ELT's location and proximity to the composite as much as anything else. After all, these things are flying all over the world, every day without trouble.
As I said before, if this had gone up in smoke while airborne, even if it had been spotted straight away I'm not sure how much the cabin crew could do to mitigate the situation before they faced serious problems with both toxic fumes and structural integrity.
What instructions have CC been given with regard to fighting fires that may involve composites?
The LHR fire service were wearing fireproof suits, safety headgear and breathing apparatus and so were able to remain in the area while they pulled down the ceiling to get access to the fire. In flight, there would be a limited amount of firefighting time available before those in the rear of the aircraft would have to be evacuated forward while the flight crew tried to get it on the ground as soon as possible.