I'm aware of the rules regarding the appropriate routing of concealed
T&E cables, i.e. within 50mm of wall edges/corners, in vertical or
horizontal lines from a visible electrical accessory or greater than
50mm below a plastered surface. Anything else would require mechanical
protection from drill bits, nails & screws and such like.

I was talking to a fully qualified sparky recently about my planned works.

He claimed that if the cable in question is protected electrically by a
RCBO at the CU, none of the above apply and that if I really wanted to,
I could run T&E in the gap between breeze/block wall and dot 'n' dabbed
plasterboard from floor to ceiling with no mechanical protection at all,
at less than 50mm depth from the surface and totally ignore the above
cable routing rules.

Is the sparky factually correct or talking brown stuff regarding wiring
17th ed regs?

If he is correct, it offers me more cable routing opportunities given
that I'm installing a CU fully populated with RCBOs only particularly
for putting in a lighting circuit for the loft on its own RCBO. (this is
deliberate in case I decide to have a loft conversion in the future so I
was planning ahead regarding circuit design for this eventuality)

Slipping the 1.5mm T&E down between wall and plasterboard from loft to
ground floor is actually an easier route than hacking access panels into
a plasterboarded stud & partition wall, drilling holes in the noggins
and reinstating the plasterboard panel cutouts and re-skimming the wall
in order to have a cable route that obeyed the rules details in the
first paragraph of this post.

Stephen.