As per subject, do I need a metal enclosure for an isolation switch
between the meter and consumer unit?

Given CUs ought to be metal cased, I would have though the principle
would apply to a switch powering the board. Yet the normal suspects only
stock ABS enclosures.

Some suspects are also selling CUs where the lid can stay down, which
again I would have thought would be against the principle of a
semi-sealed CU?