Yes the main reason this would be challenging for me is that its not easy to access the other side of the wall where the cable comes into the kitchen (the cabling was evidently installed before the plastering and kitchen units were put in).
Furthermore I'd be hesitant to extend the ring myself - when I started this saga I thought I'd take the most straightforward approach to adding the sockets I wanted to the garage, and I thought current-limiting the spur would be that. Of course even that has turned out, for various reasons, to be a lot more bother than I had hoped!