That's the heart of the matter, isn't it. What general case(s) is/are there where it would be appropriate?
A few have been suggested.
BS 7671 has a number of regulations which impose requirements for insulation. In how many of those does pressure-sensitive tape meet those requirements?
Quite a few I would expect. Note that "insulating tape" would meet the definition in section 2 for "insulation" or "basic insulation" - so it's really down to things like not being removable without a tool if it is NOT enclosed in an enclosure which requires a tool to open it.
I'm not sure what you mean by "rely on"
Easy, you quoted 412.something and said (paraphrasing) "see, that reg prohibits using tape" (by virtue of the tape not meeting a requirement in that reg) and extended that to "BS7671 prohibits using tape".
So you relied on quoting that reg for your argument.
The other way, is when it comes to section 4, how many rely on double or reinforced insulation for protection ? Very few I suspect, most using 411 "automatic disconnection of supply" as a means of protection. Hence few "rely on" the use of 412 for compliance with BS7671, using 411 for compliance. 412 is only applicable where double or reinforced insulation is used for protection.
417.3.1 says that
Simultaneously accessible parts at different potentials shall not be within arm’s reach.
A bare live part other than an overhead line shall not be within arm’s reach or within 2.5 m of the following:
(i) An exposed-conductive-part
(ii) An extraneous-conductive-part
(iii) A bare live part of any other circuit.
NOTE: Two parts are deemed to be simultaneously accessible if they are not more than 2.50 m apart (see Figure 417).
Seems pretty clear to me, and I'm not sure what difference being not in "isolation" would make
That "wooshing' sound was the point going right over your head. Someone could look in the book, read 417 and say, "look, this reg requires bits of equipment to be spaced apart. See, all those things you've put close together don't comply with BS7671. They'd be wrong, just as wrong as you were in quoting 412.whatever
in isolation (ie out of context, and without referring to the complete section) and claiming that it means the whole of BS7671 prohibits the use of tape.
What also seems pretty clear to me is that this discussion is becoming another one characterised by people not wanting the regulations to prohibit something which they have always done, and therefore refusing to accept what they say.
No, it's characterised by someone making a claim that so far they have failed to substantiate, and the rest of us being satisfied that the rags do in fact say what they say and we're happy with that.