Pffff... London on Boxing day will be quite free from traffic. You'll be fine
London isn't the monster everyone makes it out to be. Sure during rush hour you are right in the thick of it, jostling and fighting for position and gaps at junctions. However, during it's non-busy periods it isn't anywere near as bad.
All you need to do is keep your eyes open. I've driven in many countries across the world and found worse places than London during non-peak times.
