If said isolator begins to sprout additional complexity such as MCB and SPD as a shunt addition, my interpretation (Definitions, part 2) is that it still doesn’t yet qualify as a consumer unit, hence the 3M rule could be said to still run straight through it as far as the farthest CU.
the internal 6mm wiring to the shunt device is technically a "circuit" protected by the main fuse. Not sure how this fits with the DNO view.
A length limit of 3m after a 100A fuse is not a BS7671 requirement,
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