Crucially, the violence was between far-right anti-ceasefire protestors and the police. According to the article, the pro-Palestinian protestors did not engage in violence and the police were there to protect them. FTA:
More than 300,000 pro-Palestinian demonstrators marched through central London on Saturday, with police arresting over 120 people as they sought to stop far-right counter-protesters from ambushing the main rally.
Skirmishes broke out between police and the far-right groups gathered to protest against the demonstration taking place on Armistice Day, the anniversary of the end of World War One, when Britain commemorates its war dead.