For reasons I have yet to fathom, GB stickers are now not valid (on their own) on British vehicles driving abroad. Already I knew that the EU flag (if shown on the numberplate) needed to be augmented by (what was) the GB sticker[1], but now it is suddenly (apparently at the request of Westminster?) changed to the letters UK (with or without Union Flag).
And this happend yesterday, and everyone seems surprised by it so it's hard to know why - or even if anybody at the top actually intended it for... reasons..? Perhaps it was an accidental request for change, when it was meant to be a confirmation that both are valid? ISO 3166-2 still says GB is correct, I think, and UK still only 'reserved' (though it is the other way round in geographic TLDs!).
Though it's entirely possible it's to not exclude NI (not on/trivially-offshore-of GB the island), and/or to allow the exclusion of Scotland (in the event the UK stops being quite as U).
I must find out more, I've only just heard about it and nobody yet sems to know/specify the root causation. But it might cause problems (that will be blamed on various parties, naturally, as if there aren't already enough problems[2]).
[1] When still in the EU, there were a wide range of options for EU driving, including Alba/Cymru variations within certain limits, and a lot of 'EU-style' plates (even if not EU-badged) might satisfy internal rules even if not Vienna Convention-compliant (with regionally-appropriate Saltires/Crosses/Dragons/Roses/etc, for one degree or other of regionalised pride). Ecosse/Alba are quite common, for those who wish them over Sco(tland); Cym(ru) probably far far more dominant over the also allowable Wal(es), and... Well... Eng(land) is as it is.
[2] If you get a special visa to drive a lorry in the UB/GK, folks, remember that it's valid only until Christmas Eve. Which seems a bit Grinchy to me. "We desperately need you, apparently... but the moment the clock strikes midnight you need to have left. Merry Christmas, Johnny Foreigner!"