I would guess that the person was just a bit of c0ck, sounds like you did nowt wrong…
I drive into and out of manchester everyday, it amazes me the way people drive, cutting down the outside of long lines of cars to turn off, or squeezing the last inch of road out before a bus lane etc… seriously, do they get to work/home any quicker? Personally I just take it easy, stick the ipod on shuffle and leave plenty time to get to work in the first place.
As mentioned above, put the kettle on, have a biccy, and relax in the knowledge that at somepoint the person will flash & wave fingers at the wrong person.