A BRITISH arms dealer was seized by the FBI yesterday for plotting to help terrorists shoot down a US plane with a missile.The man was arrested in America after an amazing sting involving MI5, MI6, US and Russian authorities.The London-based dealer said he wanted to target George Bush’s presidential plane Air Force One, it was reported. The dealer shipped the sophisticated weapon from Russia to Baltimore — and believed he was selling it to a Muslim fanatic.But his buyer was really an undercover FBI agent — and the arms dealer is heard on tape saying the missile was to be...