I think it is true that there are a lot of fires that are electrically started, but I also suspect the majority are not due to fixed building wiring but are defective appliances.
For example, it was a fridge-freezer that started the Genfell fire, the worst in UK recent history. But the main reasons that led to such a great loss of life was lax building standards and, to a some degree, various issues relating to fire-fighting. But usually it is things like tumble dryers that are more common fire hazards it seems.
I can't speak for others but I am in favor of metal CU, it just seems sensible to have something that is reasonably tough and fire-resistant even if the risk from a well-fitted CU is negligible. But as mentioned, a decent grade of plastic that meets fire containment and LSOF behavior would be fine as well.
Perhaps they don't trust industry and/or cheap imports to keep meeting a specific grade of plastic, but "is it metal?" is harder to dodge. Who knows?