It varies - London cabs run on a combination time/mileage bais because traffic is more liekly to delay them, and the instances of being asked to wait while a fare visits one destination before travelling on is higher. Provincial taxis tend to use a mileage only meter, as point A to B journeys are the norm, and serious traffic hold ups are less common.
Actually provincial taxis use both as for the London cabs you pay mileage while in motion and waiting time while standing still ie in traffic or if you are making another stop to drop off or pick up.