Let me be straight with you, each electrician has to use their own engineering judgement to produce an EICR.
Whilst there are guides to EICR coding, these are not 'set in concrete', different situations produce different risks and this is where a skilled and competent electrician will use their judgement.
That being said, of any guides that do exist, the one most often referred to when completing tests like this, is from 'Electrical Safety First' and is entitled 'Best Practice Guide 4 - EICR'. Electrical Safety First
used to be the Electrical Safety Council
, a national body promoting electrical safety.
Their guide lists a wide range of 'situations' an electrician would encounter when checking an installation, and the code that is considered to be 'best practice' to use.
Of the question of plastic boards, their advice recommends that a C3 - IMPROVEMENT RECOMMENDED code is given for a plastic board, only when it is either under wooden stairs or else in the sole means of escape from the property.
A C3 code is akin to an 'advisory' on a car MOT. It does not constitute a fail, rather a situation where safety could be improved by having the following items rectified.
Any other areas where a plastic fuse board may be located as opposed to a metal consumer unit, would not even be considered a C3 code, rather just something to note on the EICR.