Scott wrote:
As an aside, the two screws on the lightswitch here must be earthed
(to the steel back-box) since if you touch them you get electrostatic
effect. Is this normal?

Backboxes are supposed to be earthed because a wire could come adrift and
make the backbox live, which would be transmitted through to the screws
which someone could touch. If you have a lot of static charge then you
will likely discharge yourself through the screws.

It's possible to get plastic screw covers:
https://www.ebay.co.uk/itm/BG-NEXUS-...0/293012941036
to avoid any metal parts being exposed (on plastic switches)

Theo