Its too early to speculate on the cause and will wait on the investigation.
Due to the possibility that electric vehicle batteries can reignite more than once, Ive seen the use of specialist blankets being used in America, that allow an EV fire to burn under control, once out the blanket is tightened to all safe removal and storage. I can’t say Ive heard of this in the UK but, there must be something similar used by now. However I guess such a blanket would have been useless at Luton if not caught in time before surround cars went up!