I just don't see how another country (especially now that we're out of the EU) can issue a fine or impound a UK-registered vehicle because the driver hasn't paid something as trivial as his vehicle tax back home.
Leaving the EU did not (amongst the long list of the many benefits it bestowed upon UK plc) did not absolve us from our duty (however trivial) of taxing our vehicles for use on the road. It is law that a vehicle must by duly and correctly registered / taxed in its home country, before it is driven abroad.
Whether or not a foreign agency (or indeed a British agency, when looking at a foreign car, here on ‘our’ roads) will make an enquiry as to the vehicle’s tax and insured status, is another matter entirely. What they will then do, should they discover it is not taxed or insured, is similarly another matter, too.
We (UK plc) make it easy for anyone, anywhere around the world, to access the DVLA databases. The Spanish or French policeman could just say:
“Hello guvnor, you ain’t taxed your car and it ain’t insured. We’ll fine you…. Oh, and we’ll take the keys”. They do pretty much that if you can’t produce a trivial V5 on demand and / or can’t pay the on the spot fine. The appeal of “But we left the EU, mate” only wins you a well deserved clump from a police baton, for being an arse. Nor do they care two hoots that in the UK we are not obliged to drive around with our V5 stuffed into our handbags or indeed, without very much else in the paperwork department either.
Spain, like France, Italy and a host of other western countries is short of money. The more they can raise by hitting a foreigner’s wallet, the better. We, UK plc, should do the same. But, the enforced closing of lanes of the M20 to allow the DVLA / police to check foreign registered vehicles - and very probably a stout British copper’s inability to understand anything in ‘foreign’ - with the delays caused, would only result in howls of outrage, probably starting on these pages.