I disagree - I don't see why it should be viewed as a last resort. Provided it's done by putting an FCU on the socket circuit, then running to a light switch (i.e. not the hideous idea of a plug & socket), and the cables are all in the right zones, I don't think that would be poor workmanship.It would be far better to extend the lighting circuit. Using an FCU from the ring final will work, and it's safe to do so, but from a good workmanship point of view, it should be seen as a last resort.
I disagree, but that's just personal opinion.
If there's a new wall going up, then by the sounds of it it won't, even if lifting the carpet upstairs isn't a hassle.It will probably entail significantly more work than using the existing lighting circuit,
If there's a new wall going up, then how are the cables for the sockets and proposed lighting FCU going to get there? Obviously none of us have seen the job in hand, but it doesn't sound as if it would truly be difficult to run from the lighting circuit, especially if the only barrier is a new piece of carpet!
Easily remedied with a bit of documentation. And it can be useful to have lighting on 2 separate circuits.and may also be confusing to a future occupant should they wish to isolate the lighting in that room for any reason.
A bit of documentation that can get damaged or go missing? I don't disagree with you that running lighting circuits via an FCU from the ring final is a potential option, but I still don't think it's the best way forward.