A British extremist has taken to twitter to say an Islamic State would capture Downing Street and hold executions in Trafalgar Square. Abu Dugam al-Britani, has issued a series of chilling warnings of a “holy war” in Britain, according to the Daily Mirror. Al-Britani is believed to be fighting for ISIS in Syria, and it is thought his real name is Abu Rahin Aziz, an Islamist extremist with a history of violence. He is thought to have fled Britain as he faced trial for beating up a football fan on Oxford Street in central London. In his absence he was found guilty of affray, and jailed for 36 weeks. The father-of-one, who claims to be fighting for Isis, used Twitter to warn that the Prime Minister’s official residence will become the centre of a British Islamic state. He said: “Downing Street will be a base for Muslims. Trafalgar Square is…