Zones...
- Vertical Corner-Corner 150mm
- Vertical from a wiring accessory
- Horizontal Ceiling 150mm
- Horizontal from a wiring accessory
- Intersection of Vertical-to-Horizontal formed by 2 wiring
accessories
- Reverse of wall 100mm thick where easily identified (not commonly
used but worth remembering with modern studs internal partition
walling)

There is picture in the OSG, NICEIC do one, just about everyone does
one, which makes it very clear & simple.

Note cables should not run loose on ceilings because of the 50mm from
finished surface rule.

If you have cable out of zone, or 50mm from finished surface re
ceilings, the simplest solution is BS8436 cable from Discount
Electrical (it is not listed on their website, but they do carry it in
cut lengths).

You can also use steel conduit or "mechanical protection sufficient to
prevent penetration by a nail" (very thick steel) or a wiring
accessory blank faceplate or bury 50mm from the finished surface (not
likely with Building Reg "A" chase depth limit). However for practical
reasons BS8436 1.0-1.5-2.5mm is "made for just this situation",
installed like FTE (no glands to remain accessible for Inspection &
Testing), as long as protected by 32A Type-B MCB/RCBO (ie, you can't
use a rewireable fuse or Type-D as often used for HVAC startup-surge).