From what I remember, the requirement is under a UN treaty, not something that the UK can change unilaterally.
Autoexpress article says -
"the UK Government has written to the United Nations to effect the change, stating: “The United Kingdom is changing the distinguishing sign that it had previously selected for display in international traffic on vehicles registered in the United Kingdom, from “GB” to “UK”...this change will take effect on 28 September 2021.”
I'm so glad this govt are dealing with the important stuff first.
Ironically I have just put GB stickers on a couple of my bikes because the Euro/GB plates no longer satisfied the requirements, now it appears I'll have to change the stickers.
What a bunch of sh**-for-brains wastrels, can't keep their ...
(edited - apologies, language a bit too spicy even for this broad minded forum).......... and breathe.