In the 1970s before the South side of the M25 was fully open I found it quicker to go through Central London than use the Dartford crossing at midnight to 4 am.
What really helped was one day my car broke down and put on the back of a recovery vehicle. The recovery driver was expert at avoiding London traffic. North of the river he took a route through the docklands warehouse district, through a dodgy East End housing estate and narrow back streets. Most of the route at that time of night was completely free of traffic. It's a route I subsequently took for couple of years whilst I still had to make that journey.