LONDON — The rally outside the Chinese Consulate in Manchester, in northern England, began like others before it, with demonstrators against repression in Hong Kong hanging banners and preparing to shout slogans. But the scene turned violent on Sunday after a group of men emerged from the consulate, removed a poster of Xi Jinping, China’s leader, and then dragged one protester through the gates. They kicked and beat him as police officers and other protesters rushed to separate them. Journalists at the scene caught the melee on video seen around…