"J.K. Rowling will not be arrested for comments about transgender women"
That's a famous (non-)case which completely disproves your assertion though.
"inflammatory posts on social media calling for hotels housing asylum seekers to be set alight"
I dunno, I think there's a bit of a gap between "mean stuff" and "racial hatred with an incitement to murder".
Saying a trans women is a boy is also gender hatred with possible catastrophic harm (as we know the suicide rate is pretty high on trans people). To be honest I would not be surprised at all to see people in jail for what JK Rowling said.
Ok but she wasn't even charged which suggests that people aren't going to be going to jail for saying what she said. I don't get how you take "she said some horrible stuff and wasn't charged" to "people will go to jail for saying the same thing" because there's no evidence or logical connection between the two stances.
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."