A man with a knife has been arrested near Buckingham Palace, as Britain remains on critical terror alert amid fears Manchester suicide bomber Salman Abedi did not act alone. The man was detained by police on The Mall just moments before the Queen passed by in a car on her way to an engagement with the Duke of Edinburgh at St Paul's Cathedral. A picture showed the man surrounded by officers and handcuffed as he sat on the ground, while a police officer nearby held what was believed to be a large blade that had been seized. The Metropolitan Police said the incident, at around 10.40am on Wednesday, is not believed to be terror-related. However, it comes amid heightened security and as armed soldiers prepare to guard "key locations" across London. A witness said he saw the man surrounded by officers…