I've done a bit of investigating (look, it's Sunday evening, and I'm in that kind of mood). Bus route 196 Norwood Junction (visible on side of bus in video): at that upcoming junction, the bus turns right, so wouldn't have been manoeuvring into the left lane.
This is how narrow the lanes are. The bus would have been where the blue car is.
I think the car is an Alfa 156 Sportwagon - width 1,745mm. A Routemaster bus is 2,520mm wide - 44% wider than that car. So clearly the bus is going to have to partly occupy the left hand lane, or hit oncoming traffic.
Of course, the bus will have a rear camera that will show if Foxwit's driver tried to pass a moving bus on a tight left bend.