I do begrude the black cab fares, especially at 21.00 when the rate goes up. I've found Addison Lee to be a good compromise - a professional minicab service, that has fixed fares (so getting stuck in traffic doesn't cost a fortune) and they text you when they are on their way and again when you arrive:
http://www.addisonlee.com/

(and no, I don't work for them....).

They were going to be given the right to pick up fares at LHR, but I think that the black cab mafia got BAA to back down ...