The nutter at the beginning of the video is Anjem Choudary. He failed medical school, failed law school and failed in his marriage. He has reinvented himself as the 'Chief Judge of the Sharia Courts of the United Kingdom'. As others have mentioned, he is an attention seeking clown with literally several 'followers'. The vast majority of British Muslims wouldn't give him the time of day. He lives off welfare and I'd expect our new government to find him a nice job in the near future, at which point he'll probably bugger off and join his mentor, Omar Bakri Mohammed, in exile in Lebanon.
The chest slapping thing is, I believe, part of a religious festival celebrated by a particular Muslim sect. It's generally harmless although one fervent follower was jailed a while ago for beating his 14 year old son with a chain with nails in it. Self flagellation is also practised by some Christians.
There is an ongoing problem with UK police harassing photographers. They have embraced anti-terrorism legislation with an enthusiasm only matched by the TSA's enthusiasm for the Patriot Act. There are many examples of this happening on YouTube. Note that the woman who was harassing the photographer was not a police officer. She was a 'Police Community Support Officer'. These are low cost, partly trained civilians that don't have powers of arrest. Some of them have been watching too many episodes of Miami Vice.
I think the bloke shown at the end is the new leader of the United Kingdom Independence Party, a fairly right wing party in the UK that has an anti-European agenda. He showed himself to be an absolute fool during our election last month when he was shown on live TV unable to state his own party's policies. He's no more dangerous than little Anjem.
There are serious and growing racial problems in the UK but most of them have been caused by the previous government rather than by the very small number of extremists in Britain. The Marxist wing of the Labour Party identified Muslims as an oppressed minority and set about leading them in an 'uprising' against the 'racist' majority. This has led to increasing anger at 'loony left' policies such as open door immigration, forcing school children to eat Halal meat and the gradual but steady introduction of Sharia law that the previous government encouraged.