Polluting vehicles aren't banned, just charged each time they enter the zone, via automated camera based solutions. Aim is to provide a solid financial incentive towards using more fuel efficient vehicles. Large commercial vehicles get charged £100 a day, everything else just £12.50. So not really prohibitive, but also a cost that soon adds up.
It has been highly effective, with a large reduction of pollution in the city, and no doubt the city really appreciates the additional revenue stream (even if it's tied to specific projects)