@raccoon @ret (Disclaimer: I have not lived in the UK since before Brexit, but I travel there regularly and try to keep up with the news.)

Short version:

1. EU only wants goods meeting EU rules sold in EU.
2. UK did Brexit because UK wanted to not follow EU rules.
3. EU presumes that goods produced in UK under UK rules no longer meet EU rules.
4. UK and EU share a land border on the island of Ireland, which needs to be open because History and Violence. This makes smuggling extremely easy and worth discouraging.
5. That label makes it fairly obvious if someone tries to take goods produced in UK and sell them in EU.
6. If a UK producer follows both UK and EU rules, and goes through the paperwork to prove that, they don't need the marking.
7. Paperwork is hard; it's much easier to just put the label on everything.

Long version: https://chrisgreybrexitblog.blogspot.com/2023/10/not-for-eu-labelling-case-study-of.html