But, with the qualifier of not having done an exhaustive search, BS7671 does not in many cases specify a standard to be applied. In some cases it does (eg 717.52.1), but in many it does not (eg 715.521.1 where it refers to "Insulated conductors", "Rigid cables", and "Flexible cables"). Only "flexible cable" appears in section 2 where it is merely defined as a cable suitable for being flexed.But would we have anything which conformed to the appropriate standards for a cable?
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No, but I would argue that such an assembly would not conform to any standards recognised by BS 7671 for cables.
So in answer to your challenge ...
If installing an ELV lighting system (section 715) with bare conductors, it would appear that instead of joining a section of insulated cable, it would be allowed to simply continue the bare wires past the last of the support insulators and insulate them with tape to the terminals of the transformer. Indeed it could be argued that this would be preferable to introducing additional high current connections where the transformer is very close to the lighting setup. Would I do it with tape ? Generally no, I'd use sleeving (likely heat-shrink) which would be easier and neater but have (almost) exactly the same questions as using tape.