I agree entirely that the adoption of the new scheme introduced the potential for confusion with both blue and black "changing places," and BS7671 tacitly acknowledges this fact with the label requirement. I'm certainly of the opinion that matching up where possible is a sensible approach.
However, the officially issued guidance notes for Part P indicate that "in the Secretary of State's view" one method of achieving compliance with this part of the building regulations is to follow BS7671. How, therefore, could it possibly be considered illegal to extend a red/black circuit in brown/blue when BS7671 permits this and compliance with BS7671 is also deemed to constitute compliance with Part P? (In fact the current edition not only
permits it, it
requires it, either by using actual brown/blue cable or by applying appropriate sleeving).
So, if you can reasonably avoid doing that, i.e. if you have old-colour cable in new condition available to you
How far is the definition of "available" to extend? Say I want to run a few yards along a wall to install an extra socket and have a suitable length of red/black cable in my stash, then clearly it's easily available to me. According to your interpretation, it would be illegal to use new brown/blue instead in that scenario.
But what if I don't have any suitable red/black cable stashed away? It would still be
available to me if I cared to look around, since old stocks can still be found. I could even search eBay and pay some ridiculously inflated price for 3 or 4 yards of old-stock 2.5 T&E, so it would be "available" to me, would it not? Would you still claim that it's illegal to use brown/blue to extend, because I
could acquire some red/black?
Aside from having to define "available," your interpretation of "reasonable" would give rise to the ridiculous situation in which having old cable available or not would result in the final installation using brown/blue to extend being either legal or illegal, even though the final result in both cases would be exactly the same.
You seem to be using the same rather bizarre interpretation of "reasonable" as you did about non-U.K. sockets some time ago:
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