Sparkyninja covered this in one of his
videos regarding earth back box earthing.
Reg 410.3.9(iii) states that provision for fault protection may be omitted for exposed-conductive-parts which are small enough that they cannot be gripped or come into significant reliable contact with the human body
So a socket screw does not count as something that must be earthed for fault protection purposes, even with a metal back box - and certainly not with a plastic one.
Those plastic covers are a PITA when you're trying to do an EICR - In a circuit with no cpc and class 2 switches I'd probably prefer plastic screws if it was a concern.