It would be unusual for a standard to specify an exact material - usually they just give performance requirements (passing a glow wire test or flame retardant or non-combustible sort of thing) and leave it to the designer to pick something suitable for their particular application. In this case it sounds like it's not a BS 7671 matter, more likely an equipment standard (of which there are quite a few) or even down to the enclosure manufacturer's instructions to ensure compatibility with the enclosure.
It would be unusual for a standard to specify an exact material - usually they just give performance requirements (passing a glow wire test or flame retardant or non-combustible sort of thing) and leave it to the designer to pick something suitable for their particular application. In this case it sounds like it's not a BS 7671 matter, more likely an equipment standard (of which there are quite a few) or even down to the enclosure manufacturer's instructions to ensure compatibility with the enclosure.
