I’ve seen a lot of protests make their way to Parliament. Most of them come marching under banners and flags, banging drums, singing, blowing whistles. In Trafalgar Square or Parliament Square, they set up shop and begin a series of lectures from rabble rousers, heralded by cheers and drumbeats, and then go on their way, leaving behind a sea of trash.

The protest against Labour Party anti-Semitism, organised by two Jewish groups last week, was rather different. I popped along and it was immediately clear that this was a group that doesn’t do protests. The mood was lively but directionless. The public address system was so bad that it was impossible to hear the speakers. And half the crowd had...