I've had this theory for a while that it's an Id/Ego thing. My very basic understanding of those two things is that Ego is what the world sees and Id is all your internal unfiltered behaviour. I reckon that when you get into a car, that car becomes what the world sees and you, as the consciousness safely inside it, instinctively revert to being more Id led. You feel safe and enclosed so you stop filtering your behaviour the way you would walking down the street.

As someone who has driven in different countries though, terrible behaviour is absolutely not limited to the UK. In France I experienced drivers who actually go out of their way to ruin your day. In the Middle East it was an almost complete lack of caring at all and a comically enthusiastic attitude to risk taking. In Greece everyone seemed to be ok with today being the day they die on the road and in Argentina people just pointed their cars in a direction and hoped for the best!