Makes me feel like sending a load of them to the Daily Mail...

You kid, but I imagine this will quickly be abused by bad actors to harass victims of online doxxing as ordering pizzas and (in more serious cases) swatting is.

You just don’t accept the letter, nothing to pay then

Aren't public institution obligated to accept letters from the public?

It'd be a fair campaign for everyone to return their stamps by sending them affixed to complaint cards to their local councilperson or postmaster.

It's worse than that. They introduced new stamps with barcodes and let you trade in your old lawfully obtained postage for the new barcoded stuff. They are marking some of their own lawfully obtained stamps as counterfeit and by dint of doing so stealing from their citizenry.

It's roughly analogous to having bought speed guns from an incompetent vendor which sometimes read 200 kph no matter what the speed and insisting their cops continue to use them and ticketing per normal because it would be very expensive to replace the guns.

> They are marking some of their own lawfully obtained stamps as counterfeit and by dint of doing so stealing from their citizenry

This is called "fraud" and in any properly-run country people would be doing long prison sentences for it.

Sadly, the UK is not a properly-run country.

What, you don’t think the postal court run by the Post Office will rule against the Post Office? Come on, there’s only a small chance of being sent to Rwanda in a crate if you file a case against them, hardly worth worrying about.