I would prefer to discuss a Ring Final Circuit
Yes, some odd names have been invented, so we can have an overload device which does not create a circuit as can't really have a final final circuit.
it can't (sensibly) be simply the case that anything 'plugged in' is NOT 'part of the installation
I see your point, portacabins, caravans, and narrow boats all can be plugged in, main point is TN-C-S is not allowed, but within the caravan or boat things are installed.
As to if a plug in solar panel is installed, really not sure, whole point is it can be moved home to home, so common sense says it is not installed, but to unplug and plug back in every 27 days so a G98 is not required, seems to be stretching it a bit.
As to plastic, they mean it can bend, so how do we get hard plastic? And a boiler with change the state of something from liquid to gas, so how do we end up with a central heating boiler, furnace would seem a better name.
OK we know what PAT testing is, and would not call it portable appliance testing testing. It is only when the word is used in a law or regulation that we have a problem.
So 30 MPH in relation to the earths surface is not required, we take that as read, fact I am doing around 330 MPH when standing still, is ignored.
But the "The bi-colour combination green-and-yellow shall be used exclusively for identification of a protective conductor and this combination shall not be used for any, other purpose." full stop at the end, is often ignored.