TfL and Westminster City Council talk about improving the air quality on Oxford Street. However, to talk about it in these terms is simplistic. Since the traffic is merely being moved, all the pollution from emissions will simply move with it, resulting in the areas to the north and south of Oxford Street having worse air quality. This isn’t fair to local residents.
Banning vehicles from Oxford Street will likely result in some less pollution on the street itself- although there are other ways to do this, given zero-emissions taxis and buses are increasing in availability.