This type of behavior from motorists usually results from ignorance (i.e. not knowing that cyclists have a right to ride on the road and control a lane) or impatience (i.e. someone is late and no one else matters to them). Neither of these things is something that you can really resolve with the motorist at the time. If they are behind you and honking, then at least they see you.
I would advise against escalating as well. I sometimes also flip honkers off instinctually, but this can lead to people trying to run you off the road, or worse. It can be a hard pill to swallow, but just wave them by and try your best to ignore them. Throwing something at the car could result in a few hours of jail time. I just take down plates and call the police if anyone gets too aggressive with me.