Earlier quoted context omitted.
All standard British stamps now have unique Data Matrix codes on them, which means you also have to source your stamps anonymously.
Wait what? If you go to the post office and pay with your debit card, how exactly do they figure out who you are based on the stamp?
If they don't do that, if they meet the stamp along the letter's journey, they can scan the code and check which batch it's from, and there could be a database of which post office got which batch, and then it's a matter of checking that post office's purchases/security cameras.
If all stamps are indistinguishable from each other, then you could've bought the stamp months ago on the Isle of Skye and used it in London, they wouldn't be able to tell the difference.