Passport stamps are required upon entering and exiting Schengen (to be replaced with an electronic system - ETIAS - in 2022). Therefore, having your passport stamped on exiting Spain and then proceeding through France by road makes no sense whatsoever. Surely a misunderstanding?
Therefore, as far as I understand it, a UK or GB marked car that only has the UK/GB on the number plates is actually not displaying the code in the correct format since 1/1/21.
I beg to differ.
The latest forms are in 5 sections.
I failed on the first section when it asked me to enter my phone number.
It was in a "drop down combo" box as GB44 (not 044 or +44).
Hard enough to find using a computer but on a mobile phone finding the entry on a small touch screen was impossible during the panic at the airport.
That example is easy now but a PIA on the first time that you come across it.