That is true, but I suspect that the next amendment to, or edition of, BS7671 may well 'catch up' by explicitly adding these things to the existing list of 'exclusions'.
Mind you, I have to wonder why people don't seem to get excited about IEC connectors which, when used in some ways, have the 'fixed' part as the female one ('socket') serving as an 'output'.
You're attempting to apply common sense, but I'd suggest that it is not at all easy to do so in a manner which is at the same time 'comprehensive enough',consistent and also free of potential 'loopholes'.
Again, you are attempting to apply common sense, and it's far from straightforward ....
I think that most of us would probably agree that none of the things we normally plug into 'mains sockets' (kitchen appliances, TVs, IT equipment, vacuum cleaners, hair dryers, tools etc. etc.), including their supply cables and plugs, count as parts of the electrical installation.
However, conversely we run into the issue highlighted by this thread. If a string of wall-mounted sockets are supplied, as a 'fused spur' from some circuit, then I think that many, maybe most, of us would, per what you imply (and what App Doc P says), always be inclined to say that those sockets were 'part of the installation', even if that 'spur' was supplied by a BS1363 plug/socket.
However, I don't see that anyone could/would describe an N-way 'extension lead' which plugged into a 13A socket as being, per se, 'part of the installation', and I suspect that many (most?) would say that that remains the case even if the socket outlets at the end of the extension lead are 'attached' to a wall/whatever.. As has happened in this thread, someone is therefore bound to point out that the two situations are electrically identical, rendering it difficult to produce a consistent 'rule' regarding what is. or is not, 'part of the installation' in these situations.
