I don't think that's always going to work without some further detail or qualifications. You say that 'what it's supplied by' is irrelevant, but I'm inclined to think that it's very relevent, since your 'cable from a JB' criterion seems only to really work if that JB is fed from a 'final circuit'. If you will accept that a Henley block is merely a type of JB, I don't think that anyone in their right mind would doubt that new wiring from a Henley at the origin of the installation constituted a 'new circuit', would they?
This is the interesting one in terms of how this discussion started. You say 'not a new circuit'. Presumably you would still say the same if, for whatever reason, I interposed an FCU in that circuit? But are you saying that if, instead of an FCU, I interposed an MCB (say a 6A or 10A one, in its own enclosure), then it would suddenly become 'a new circuit'? If so, why - is there that much difference between a BS1362 fuse and an MCB of similar In?
Kind Regards, John