Decisions seem to be based on public pressure or context. The man posting the picture of poppy's burning was arrested because it was Remembrance Day. Because of common law these cases are each setting precedence. Although many may believe it wouldn't go farther than this. There's nothing stopping judges ruling that publicly disagreeing with a political party or common held belief is a form of defamation.

If people rally around a case the police and courts are forced to react with harsh sentencing. Morally these people are in the wrong but a moral justification is different to a legal one.

Paul Chambers was found guilty for sending menacing electronic communication. He tweeted that he would blow up Robin Hood Airport when it closed after heavy snow. I've seen worse sarcastic tweets than this in my stream before, after the listed cases these tweets could result in legal action.