Best thing to do is go to the nearest depot and ask to speak to a supervisor/manager, preferably as soon as you can.
I did that when an Arriva bus passed me within inches (literally) on a completely wide open, empty road and within a week they'd called me back and discussed it at length over the phone, taken the footage off the bus, reviewed it and punished the driver. Sometimes putting a face to a complaint helps because they know if it happens again you're 100% going to complain again and they'll know it's the same person. I'll be honest, I've had no problems with the drivers of the bus route that nearly took me off - they all give a wide berth.