The law is quite clear: if your behaviour leads to or is likely to cause a breach of the peace - then you are at fault and could be prosecuted.

I think you are missing the point. Those hecklers stirring up trouble were the ones likely to cause a breach of the peace, the street preachers themselves could just have been ignored.

If you take your way of looking at it to it's logical conclusion you could have a situation where if one person living in a street was the subject of hatred by all the other neighbours and every time they left the house, a crowd gathered and there was a rumpus, the person could be arrested for leaving their house.

What we have is a tacit admission that it's okay to silence an opposing point of view by using violence and not by reasoned argument.

We can see what that leads to, who is committing the breach of the peace here, Milo for accepting an invitation to speak or his opponents?