as evidence of work having been done after Part P came into force.

Indeed the guv'mint cocked up. Hence new colours become available and
legal
before Part P was implemented and so cable colour is unable to be used as
proof that work should have been done under the new regulations.

A good reason to buy a few reels of old colours now before everyone runs out
so I have "stock" for jobs over the next few years and there can be no
question about when the work was done later ;-)

My local electrical wholesaler had no old colours left when I tried but B&Q
had plenty.