Iran is believed to have drawn up a blueprint for terror attacks involving sleeper cells and proxy groups in Britain and Europe in the event of the assassination of its leader, experts and security officials have warned. The death of Ayatollah Ali Khamenei in an assault by the US and Israel on Tehran and other Iranian cities has already prompted a review of the UK’s terror threat level, as the West braces for a backlash from the Islamic Republic. MI5 warned last year that Iran had been behind 20 potentially lethal plots on UK soil in the previous 12 months....