You can find taxi ranks outside many railway stations, some large stores and at other obviously busy places, like shopping centres.
The famous black cab is, well, famous and usually black, though possibly covered in advertising.
Taxi drivers complete a course called The Knowledge which is notoriously tough but gets them to learn and understand the roads and destinations in London. So you rarely see a black cab driver using GPS.
Although there may be differences in model designs, London black cabs are basically the same. The only variation is that some have automatic opening and closing of passenger doors, and most do not. If you get one with automatic doors, don’t force them. Let the doors do their stuff.