What I'd like to do is to go under the stairs, drill out through the back
of
the consumer unit and then chase a channel out horizontally, at that
height,
around that wall, then going vertically up to socket boxes positioned on
that wall above the worktops,

In a kitchen, I would run horizontally directly from socket to socket at the
above worktop height. This reduces the circuit length. It is allowed by
regulations, although I would only consider it in the kitchen situation.

Your solution is only acceptable if the horizontal sections are surface
mounted on the wall (i.e. behind the units). If buried, then they would need
physical protection.

Christian.