Streisand effect already kicked in. So that people here know exactly what the arrestable offense was, the joke was "What's the difference between Mark Bridger and Santa Claus? Mark Bridger comes in April." In fact the joke wasn't even made up by that guy, he just posted it on some particular facebook page related to the search for the missing (murdered) child? Obvious remedy given that it was offensive in that place? Wipe it and ban him.
Tasteless black humor warrants prison in the UK? Just added it to my list of hellholes I'll never visit. And this is one case where the EU will probably be of some good use, as on recourse its courts will probably throw away this insane judgement.