To which organisation would I report a bus driver for what I consider
dangerous driving?

I got on a 45 bus at the Chancery Lane end of Grays Inn Road on Thursday
(19 Aug) and went towards Kings Cross. The driver cut up several
drivers while switching lanes and seemed to be challenging a Transit van
(I don't know that the white van men were entirely blameless though) and
later another white van further along. At the traffic lights before
turning right to go to the stop along the side of Kings Cross station,
the driver accelerated and drove diagonally while shouting something at
the van driver to his right, and then slammed on the brakes while
laughing to himself.

I'm young and strong and was partially ready for a jolt, but was still
almost thrown from my seat. The bus was empty but for a few passengers
upstairs (I was downstairs towards the back), but had it been full or
had there been an elderly passenger, I'm confident that passengers would
have been injured thanks to the unnecessary acceleration and braking.

The clock inside Kings Cross station read 10:08 when I left the bus and
entered the station, and the number on the yellow card beside the front
opening doors was '405Q'. I was travelling to Kings Cross to meet an
elderly relative who had taken the train down to London from the north.
Our onward journey didn't require a return journey on the same bus
route, though had it done so, I would not have boarded a bus with the
same driver - he needs taking off the road if he is unable to control
his temper while in charge of a public bus on a busy road.