I thought the issue might have been if someone breaks into the cable
they wouldn't know what was done at the ends? (ie, how I allocated the
single phase use of the 3 phase colours, or is there a rule I've
missed?)

The same would apply for using a twin and earth as a feed and
switchwire, the black will be live. I think yoou're meant to check
what the cable does *before* you start chopping it up and connecting
things to it.

Isn't there something about leads for portable appliances where they
can use whatever coloured wire they choose as long as the plugs are
moulded on each end?

Have to say I've not heard of that one, logically though if you cut
the plug off a lead you would want the correct colours there to be
able to wire up a new one. But then again, some things defy logic!
...

SJW
A.C.S. Ltd.