When it comes to an EICR, the inspector has to decide whether he/she feels that a plastic CU means that "improvement is recommended" (C3) or whether 'remedial action' is required "urgently" (C2) or "immediately" (C1) - and that's where the differences of opinion and variations arises.
You missed out the other option I was advised by a representative of the IET. If the CU isn't in an escape route, then don't even mention it. But, that was given as a personal opinion and my requests for an official position supported by the IET was ignored.
As to the example given of gas pressure, that's a fairly simple one. The analogy in an EICR is you measure the loop impedance of a circuit, look at the relevant table, and if the loop impedance is too high to trip the OCP in the required time then it's a fail (a matter of judgement whether it's C1 or C2) - if it's low enough then you do nothing but note it on the test results page. For a gas appliance, you measure the gas pressure under full load - if it's below the manufacturer's specification then it's a fail, otherwise it's OK. I'm not sure how much leeway the tester has in terms of issuing an "at risk" (roughly a C2), or "immediately dangerous" (roughly a C1).
Most of what is checked for a gas safety check are similarly easy. There are laid down rules on siting of flues for example - so a flue either meets the rules or it doesn't.
I did say "most" there. There is some judgement in terms of things that are NCS (not to current standards), and there's some variation there. Some regs are explicitly retrospective - for example flue routing and it being accessible for inspection (many properties were build with flues boxed-in or otherwise hidden, and these needed works doing or failed inspection). Some would appear not to be - I know that in our flat, we can't fit a new boiler and use the same flue location, but as long as the same boiler stays there then it's OK - but I think there's also an element there of the extra condensation you get from a condensing boiler which makes the current location unsuitable.
Another guidance says it should be C2 (potentially dangerous and to be rectified) if in understairs cupboards. Someone has just made up that totally out of the blue.
If under WOODEN stairs actually. And I don't think it was completely out of the blue (or pulled from someone's backside).
The intent behind the guidance is clearly along the lines of "a conflagrating plastic CU shouldn't stop someone getting out of the house". if it's under the wooden stairs, then there's a logical risk that the fire gets going and the structural integrity of the stairs may be impaired before people have got out (and the FRS have checked that everyone is out). Similarly if the CU is directly in the escape route.
It's clearly been done as the IET have recognised the absurdity of telling people that the shiny new CU they had fitted 2 years ago now needs to be replaced because it's become dangerous - it obviously underwent some transmogrification at midnight when the new regs came into force, a bit like
Kevin turning 13 But rather than properly admit the error and revise the regs, we have to put up with "not officially endorsed" guidance issued in the name of the IET to shift the problem onto everyone else.