From the wiring regulations point of view of prescribed (safe) zones, at what angle does a wall become a ceiling? I.e, what are the rules for skeilings, A-frames, etc?

The regs don't make it clear, I'm afraid, they just refer to walls and ceilings without defining any angle that makes a wall a ceiling, or vice versa.  Here are the two pages from the OSG that cover locations:

 

 

Assuming it's a typical Scottish 45 degree "ceiling"?

 

Treat it as a wall, you will have a safe zone horizontally and vertically from every light fitting and 150mm from the corners.

Yes, 45° skeiling would be one case. To be safe, I suppose, you'd not want to lay cables in the safe zone at the top of the vertical part of the wall just below where the skeiling starts but, at the same time, you'd not want to go banging nails in there, either.

 

I was actually thinking, though, of the 60° sloping roof on my A-frame house. For the most part I envisage wiring dropping straight down from the lofts above to any fittings on that roof but there might be a few odd cases where whether it needs to be treated as a ceiling or a wall might matter.