eggcup wrote;
"It's obviously bonkers - for example if you are staying 6 months in Spain and 6 months in the UK every year, are you supposed to change your plates over on the border each time? "
No you are supposed to stop at the non-existant border , & fill-in a certificate stating that you are an " extended vacationist ". This then absolves you from any requirement to comply with residency/registration requirements.
I have only known of one person who has actually managed to do this.
The only place you are likely to obtain it at the non-existant borders these days is the Aduane/customs clearance areas that are usually just before where the borders used to be.