London doesn't have a grid structure. When there's a closure or delay on one route, it's not as simple as choosing the next road over. GPSes vary in their quality for routing diversions, but in a maze of one-way streets, turning restrictions and temporary / permanent rerouting in effect, a human is always going to be better. GPS maps are often out of date as London is constantly changing with rat-runs being closed down and new railway lines being built.
And "The Knowledge" doesn't provide the ability for taxi drivers to psychopathically become aware of delays in traffic on a given road. I've never noticed any particular difference in the amount of time I've been stuck in traffic when travelling in black cabs or in taxis with drivers using a GPS.
