If 1 in 100 people halved their car journeys, there would be 25 million fewer tons of pollutants released into the atmosphere per year - just from 1 in 100 people.
If you can't do without the car, at least make sure it's as energy-efficient as possible. Why not organize a car-share with people from work? Journeys in the car on your own are so wasteful, especially if everyone's doing the same thing: you use 13 times more carbon monoxide and 5 times more hydrocarbons (fossil fuels) taking the car alone than you do the bus.
And don't forget the Tube, tram and your bike - you don't need the car as much as you think you do!