It may be my North Face jacket and American accent, but every time I've asked a minicab driver for a price quote, it was either the same as or actually more than the black cab price. Maybe I could get the price down if I stood there and negotiated for twenty minutes, but it's easier to just get a black cab. For longer trips, black cab drivers are often willing to negotiate flat rates below meter; I don't know if that's allowed or if they're dodging taxes or whatever.