I would add words of caution alongside Grub et al.
After such incidents I often fantasize about stopping the car and showing them that out of that fancy metal cage, my martial arts ass can break bits of them as their car could break bits of me. In my mind cars burn with their occupants strangled by their seatbelts, or a single well placed anti-tank missile from the concealed launcher in my top tube showers their car all over central London... etcetera ad nauseum.
However, I restrain myself to a simple 'why' open palmed arm gesture and maybe a shake of the head, and that is only if the occupant of the car is on their own and unlikely to be carrying a weapon. i.e. when passing thorough Brixton, I keep my head down and the power up. The arm gesture is enough to trigger some kind of primal rage in a certain type of driver, and I have watched many motons screaming off up the road arms waving as they bring themselves closer and closer to a heart attack.
Why?
Because my safety is Priority 1. Whatever they did to me, if I escaped injury, I would rather keep it that way than get beaten up or have my heart fluttered by the surge of adrenalin that accompanies confrontation with an irate driver. I'm not afraid of the average Joe, they would probably be mouthy at most, maybe a little shove that a nasty arm lock would dissuade from going any further. I'm worried about the mental pikey who wouldn't stop until I was nearly dead.
Chances are, if something happens to you on your commute, it was done by someone else on theirs - so you are likely to come across them again. Don't submit to them meekly, but if you chose to act, you must take it far enough that they are too scared to piss you off again. Personally, I don't want to cross that line.
Take Nuttys' advice to me - get off the road, have a sip of water and calm down for 2 minutes then get back on your way, fresh and as Zen as possible.