Could be on a warning label attached to the cable near the plug.
Yes, you realise I am sure I am being pedantic, and I don't believe it's OK to use an extension lead, but if we look at BS7671 how many times have we seen a small change, as someone has decided to bend the rules to suit what they want to do.
I remember being told in a health and safety meeting I was forced to attend, all items must have a PAT test label, and if not, we must stop using the item straight away, no excuses, so I walked to the lead set supplying his computer which was running the PowerPoint presentation, and pointed out the lead had no label, the computer did, but not the lead. So he should turn off the computer now, and having been alerted about the error, should check all other computers in the building.
He did not know what to say, so I continued that I could not PAT test the computer without the lead, so in real terms the lead has been tested, in the same way as a mag mount drill would have one label for drill and mag mount even when technically two items.
So some common sense should be used. And if the 30 people in the class were office workers, and they all returned to their offices and all took 5 lead set each to be PAT tested, it would take all day to test them all with the PAT test machines being used at the time. (Top of range Robin)
As to plug-in solar, we already know you can buy all the bits included in the kit on their own, but rules say, must be sold and installed as a kit, there are many items we could raise to show the law does not seem to make sense, but use a FCU instead of a BS1363 plug and socket, or even use a V2G EV plug and socket, and a different set of rules apply.