LONDON (Reuters) -British police arrested a man armed with a knife who confronted two Ministry of Defence police officers near Prime Minister Boris Johnson’s official residence in central London on Monday. Police cordoned off Whitehall, which is home to several government departments including the Foreign Office and Ministry of Defence and leads to Johnson’s Downing Street office and residence, for several hours but it was later reopened. An investigation was launched following the incident at Horse Guards Parade, a ceremonial parade ground which overlooks St James’s Park, police said. “At aro…