> Have there been real any cases of that?

I don't think there has. There's definitely been people locked up for tweeting[1] but that was less "mean" and more "overt racism".

(Before the Public Order Act 1986[2], Section 5 was amended in 2013, people were able to be charged for "insults" which was clearly ludicrous.)

[1] https://www.theguardian.com/uk/2012/mar/27/student-jailed-fa...

[2] Yet another thing to thank Thatcher for. Who, ironically, is probably a hero of the kind of people who go around saying things like "the left will lock you up for saying mean things."

What about this one? https://www.northants.police.uk/news/northants/news/in-court...

After stating that she was “completely sure that he knew racial hatred would be stirred up” as a result of his “utterly repulsive, racist and shocking posts that have no place in civilised society, including on social media”, HH Judge Lucking sentenced Kay to three years and two months in prison. Detective Chief Superintendent Rich Tompkins said: “I hope this case sends a very clear message to people who think they can hide behind a screen and publish hateful, racially prejudiced posts without consequence.