It's no different from simply bending over a spare core of a T+E if ever you had one. All cores should be put home into a terminal of some sort. That included the earth.
I guess you could interpret something in the selection and errection section, but I am guessing there is not a reg which states "do not sleeve and bend over the CPC on a switch with only a single cable".
It has long been taught this way, and I know the NIC take a very dim view of folk who do it this way. I know the NIC are not an enforcing body, but as a member, I go by their recommendations.
One key difference is because earths are done with sleeving you can make the sleeve longer than the conductor. This means there is no exposed metal to short to anything unlike with your spare core of T&E scenario.
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