My bug-bear !!! I always extend the 'active pair' if it's getting down to the cable butt. It only takes seconds. Equally, removing the outer cable-sheathing takes literally seconds to achieve, thus ensuring what you are calling the 'active pair' (commonly known as the 'feed'), is sufficient in length.
As you aren't supposed to be messing with this side of the network ..........a) I wouldn't request OR return to put your circuit onto another spare pair of wires in the black cable, and b) They wouldn't do that anyway, they'd just carry out the action/s I suggested at the start.
If you're hell-bent on making the kitchen as the master socket, then could you not cut the pair further up on the existing wire, feeding the upstairs master socket ?? Hard to put across in words, but instead of cutting it at the crimps like you say, cut it nearer the butt of the cable going to the socket upstairs and then you'll have enough to patch through to the 2 wires feeding the socket in the kitchen. It means you'll have 2 crimps on each wire (4 in total), but you'll be able to achieve what you want.