To remind you of a couple of recent cases:
Twitter user Paul Chambers received a £1000 fine for something that was obviously a joke.
A couple of kids on Facebook got four years for inciting a riot. Various other people were charged, but not found guilty.
Since Jeremy Clarkson wants me to be taken outside and executed in front of my family (and let’s not forget that public sector workers who don’t work in comfortable university offices are frequently subjected to violence in the course of their work), I wish Unison the best of look in finding something for him to be charged with. Prosecuting people who use a major public platform (and you don't get much bigger than primetime BBC1) to promote violence against a group already disproportionately affected by violence sounds an entirely sensible use of CPS money in my book. Meanwhile, the PM thinks that this is absolutely OK as long as the speaker "didn't really mean it". That'll be a great comfort to the next parademic who gets his head kicked in.