If one looks at skirting trunking
the cables are in the safe zones. It is acceptable to run horizontal between sockets.
553.1.6 A socket-outlet on a wall or similar structure shall be mounted at a height above the floor or any working surface to minimize the risk of mechanical damage to the socket-outlet or to an associated plug and its flexible cord which might be caused during insertion, use or withdrawal of the plug.
This is the limiting factor traditionally we did use skirting mounted sockets but these must be high enough to use without damaging the cable. Although you can get special plugs
this does not really satisfy the regulation as one really must expect standard plugs to be used.
Also mounting sockets upside down is not really an option. However there is nothing to say cable must be berried and this type
of cable clip has be traditionally used to hold twin and earth to top of skirting. After painting sometimes hard to work out what is cable and what is skirting using a round cable either flex or Ali-tube it can look like the beading of the skirting.
It could be claimed putting cables inside any covering
is burying the cable as it hides the cable from sight.
Why 150 mm from corner formed by two walls or ceiling is considered safe zone but 150 mm from floor is not I don't know. If you fitted a socket box in the skirting with say a FCU then from that went up into a socket this would be permitted as the FCU in the skirting means the skirting is now considered as a safe zone.
Where it becomes a problem is with this
one has to ask is it obvious that it is skirting trunking and not just skirting as since the socket is raised above the trunking cables in the trunking are no longer in a safe zone.
One coat of paint and there is little difference between skirting and trunking if one was for example to put trunking on the floor to wall corner then place plastic skirting of same thickness above it may look as if it's all skirting in which case although it may comply it could still mean others would not realise what it was. And lets face it this is the whole idea making it so it does not look like trunking but more like skirting.
So we have to start to look at common sense rather than pure regulations. So whole idea is to make it so people in the future will not damage the cables. With a skirting made from a material which no one would want to drill or damage be it shiny plastic or vanished oak panel then cables behind the structure are reasonably safe. But behind painted wood or plastic be it standard skirting or trunking then in the future people may consider knocking a nail into the structure so it's not safe.
Personally where sockets are mounted touching skirting I would expect to find wires behind the skirting but mounted just one inch above the skirting then I would not.
Finding trunking to comply with BS EN 50085 and still looks good is hard. Most is rather industrial stuff. As to capping once you use metal capping then it becomes a wall or partition the internal construction of which includes metallic part which opens up a bag of worms including earthing a metal covering.
I have argued with plasterers who wanted metal capping as easier to plaster over but I insisted had to be plastic.