Light switch location

Well side of the face plate rather than the face. But yes, bottom (or top depending on which way round you install it).

Note that if your bookcase is permanently installed then you could argue that makes the secondary containment of the wire joints anyway. So you join your wires (with insulated terminals) then the bookcase itself forms the insulating enclosure that protects those.

When I built and installed this:

It covered a socket on the lower right alcove wall. So I crimped an extension onto the wires, and relocated the socket into a dry lining box set into the right hand side wall above the worktop. I did not feel the need to provide an additional enclosure around the junction point since the lower drawer unit did that anyway.

John,

I had wondered (in fact I think I mentioned it somewhere in this thread) if the fact that the bookcase sides would be a flush fit to the wall meant that I didn't need a faceplate.

Or side, if I shift the back box through ninety degrees...

The CLICK/Scolmore 20A flex outlet plate is 9.5mm deep and has a front or bottom cable entry point.

The incorrect drawing here will not fill you with confidence but I have measured one and it is 9.5mm for the full depth (along with most other CLICK switches, sockets and fused spurs

and the switches and sockets are drawn showing the correct dimensions

I would say 1mm to meet the regs.

Belated thanks for that, and to all for the advice and help.