Lobster wrote:

What would you consider to be wrong with (a): just the fact that it's
touching the pipe (why, cos it's already electrically connected to it?)

Wiring regulation 528-02-04 is the one usually cited as disallowing this
sort of thing:

"Where a wiring system is to be installed in proximity to a
non-electrical service it shall be so arranged that any foreseeable
operation carried out on either service shall not cause damage to the
other."

IOW a plumber may disturb the wiring, or an electrician the plumbing.
So only method (c) complies with BS 7671, IMHO.

--
Andy