EricP wrote:

"More than 50mm" would be over half way through many of my walls and
dangerous from the other side.

No, it's permitted to run cables in walls at less than 50mm depth when
they're in the "standard" places: vertically or horizontally in line
with visible electric fittings, and within 150mm of top and side
corners. So your plans to cable new sockets on your internal walls are
fine; just run the cable in the "conventional" routes. Otherwise, few
Barrat boxes could have sockets on their internal walls!

Stefek