The standard way to hail a cab in London is to lean in the passenger window when the cab pulls up to you, tell 'em where you're going and then they unlock the doors and you get in.
A couple of times I had people say they weren't going that way, etc. Once I said "I know you have to take me!" but there's not a lot you can do when they say "feck off you cahnt" and drive off. By the time the next day rolls around it's just not worth the effort to file a complaint.
On the other hand, as many times as that happened, I had cab drivers say that they were on the way home, heading in the same direction as I was, and so they stopped the meter early and I got a discount. I also got meters stopped early a few times for showing them a totally awesome short cut near where I lived.