Two men were stopped boarding US-bound planes at Heathrow days before Britain's terror threat was raised to "severe".News of the incidents came hours after Home Secretary Alan Johnson lifted the threat level amid fears that al-Qaeda is planning an attack. The new level, which means an attack is reckoned "highly likely", is second only to "critical".Security sources say an Egyptian was stopped last Saturday as he tried to board an American Airlines flight to Miami. A man from Saudi Arabia was banned from boarding a United Airlines flight to Chicago the next day and sent back to Saudi.The incidents and...