French photographer Genaro Bardy tells the Daily Mail:

It turns out there is a moment in the year when streets are totally empty, when people are home or away, when nobody is partying all night long. That moment is Christmas. Between London Bridge and Piccadilly Circus I saw no one, no cars. The city was truly asleep. Very long exposure could erase people walking, although imperfectly, but car lights would ruin any picture. Between 11:00 pm and 4:00 am, it’s pretty hard to find 20 to 30 seconds slots without any cars. But starting at 4:00 am it’s truly magical, almost no one is in the streets.