"John Rumm" wrote

Or run the cables on the surface: AIUI it's only for cables buried 50mm
the RCD becomes necessary.

Yup, also true.

--
Can't you simply earth the conduit at the point that it enters each back box
by linking to the socket earth?
In this way you cover the vertical drop that is likely to be drilled through
and continuity of these to the horizontal under-floor runs is not an issue.

Phil