The 12K figure is the total figure for arrests for Malicious Communications offenses. It's not a figure for social media posts specifically. The vast majority of these arrests have nothing to do with people making edgy social media posts. They're arrests for personal threats, harassment, stalking, etc. etc.
I think a lot of people assume by default that other European countries have fewer (or less egregious) arrests for social media posts just because the American right isn't boosting the relevant stories. But if you Google, you'll find some pretty wild instances of arrests for social media posts in lots of other European countries. Here are some examples.
In Spain you can be arrested for insulting the King: https://en.wikipedia.org/wiki/Pablo_Has%C3%A9l [No, he didn't just insult the King, but that is one of the crimes he was charged with.]
In Germany you can be arrested for being rude about civil servants: https://www.aa.com.tr/en/europe/german-police-raid-home-of-s...
In France, you can be convicted (not just arrested) for posting a non-doctored photo that makes some policemen look bad: https://www.amnesty.ie/france-criminal-conviction-for-a-twee...
In the Czech Republic, you can be convicted for making rude posts about Ukranians: https://www.newsendip.com/in-czech-republic-two-men-guilty-o... And if you're a communist, that's too bad, because it's illegal to express support for communist ideology (on social media or anywhere else).
In Italy, you can be fined for mocking the President's height: https://www.reuters.com/world/europe/italian-journalist-orde...
All of these (with the exception of some of the non-King-related charges in the Spanish case) are pretty clearly instances where someone would not be arrested – and would certainly not be convicted – in the UK. But you won't find out about these cases from Elon Musk's X feed. And of course, people do get arrested and jailed for social media posts even in the land of the free, from time to time:
https://edition.cnn.com/2025/12/17/politics/retired-cop-jail...
See also this article for some more context on the '12K' claim: https://www.standard.co.uk/news/tommy-robinson-uk-speech-cla...