That is an organised, scheduled protest in London, about longstanding actions in Gaza. Why should it stop, particularly?

You (with the person who tweeted) are misrepresenting both this protest and the opinions of the people who are protesting.

People on the ground at many of these protests are saying that enough people are celebrating the attack. In London they were attacking the police. What % need to support the attack before it's a problem?

A higher % than "some people who hate the protesters say they definitely heard it happening."