It should be either acceptable or not and existing plastic consumer units should either be acceptable or not. I'm obviously missing something.
I think we all agree on that.
The problem is that "there was a problem, something needed to be done, this was something, so it was done". The root cause is, some of us suspect, poor workmanship during the mass meter change program that's being foisted on people. Meter fitters moving cables, and not checking connections afterwards, can leave bad connections where the meter tails enter the terminals of the main switch - causing heating and in many cases, a fire. Thus we believe the LFB (London Fire Brigade) called for CUs to be "non combustible". Constrary to popular belief, that does not mean they have to be steel cased - but the reg was so badly crafted as to not define what they mean by non-combustible, and stated that ferrous metal is an example of non-combustible material. So in the absence of any definition to work to, the only material you can be sure of compliance with the reg is steel.
But there's a bigger problem in that the reg just says that CUs must be made of non-combustible material - but it doesn't say that they actually have to be able to "contain" a fire that starts within as there's no requirement for lids to self close etc. And as stated, nothing is said about existing CUs.
What is stated is that when inspecting/testing an installation, it should always be to the current standards - no-one is expected to know all the previous versions of standards the installation may have been designed/installed to. So a plastic cased CU is non-compliant with the current standard - there isn't any argument about that.
Where all the "discussion" comes from is what to do when EICRing such an installation. It's non-compliant, but BS7671 explicitly says that non-compliant does not automatically mean not safe. And different organisations have issued their ideas of what the implications of non-complaince are - some have recommended coding C2, some have recommended coding C3, some have recommended that the coding depends on the situation, ...
I've had email correspondence with someone connected with producing BS7671. The guidance I had was that if it's combustible and in an escape route then code it C2, otherwise either code it C3 or don't even mention it. But that is "personal opinion" and not official guidance. In part, I think it's driven by a realisation that telling potentially millions of owners that the new CU has to be replaced again would be "unpopular"
The bodies who's names appears on the front of the book have declined to come out with definitive official guidance. IMO that's inexcusable as it leaves the whole industry in this big swamp of uncertainty - as you've witnessed in this thread. I've noted that the draft of the next version of BS7671 does not change the wording at all for this reg - and I've posted a comment to the effect that they really need to, at the very least, define what non-combustible means (preferably by referencing an existing standard - it exists). If they referenced a standard for combustibility then manufacturers could potentially declare existing products as compliant (if they are).
And for good measure, there are arguments for non-conductive enclosures being electrically safer WRT certain risks.