Reading the Draft for Public Comment, 421.1.200 is a sub-section of 421.1.5 and as such, seems to apply only to "electrical equipment in a single location containing flammable liquid in significant quantity".
Even if not, the requirement is for Switchgear assemblies including consumer units to be manufactured from non-combustible or NOT READILY COMBUSTIBLE material. This is further defined as meeting a 960 C glow-wire flammability test as defined in BSEN 60695-2-11.
I am certain that enclosure manufacturers would have selected plastics that comply, certainly the IP65 Enclosures I import from China do. Its material is described as "High Intensity ABS / PC plastic, with refractory additives for fireproofing and self-extinguishing characteristics, as well as additive to produce good UV stability".
As you sit at your computer, the chances are its case is manufactured from a similar plastic compound.
Reg 421.1.6 seems to give an opt-out clause, stating that if no product standard exists, then the enclosure shall withstand the highest temperature likely to be produced by the electrical equipment IN NORMAL USE.
To me, this a far cry from banning all plastic boxes.
SBS Dave