This is dependent on the positioning of the car. If the car is on private land and aforementioned land has a curb, then "weeing" is perfectly acceptable. If the car is parked next to a curb and the curb is, maybe, on Oxford Street and person "wees" on back wheel, then this will be construed as being a "pervert" and person will....most likely....be arrested as most people don't like people "weeing" in public....although doing it in Deptford isn't that bad as that part of London is a bit of a toilet!