Thanks to Kristi's recommendation from a long time ago, I now only use Ray Skinner:
http://www.london-transfers.com/ Never have I dealt with a more professional, friendly driver... it's a bit more (about £50 from Heathrow to Finchley), but, man, you feel like a star... he's showered, suited, holding a PRINTED CLEAR sign with your name on it (a real blessing when you're coming out with a crowd, and facing an enormous crowd, half holding signs with handwritten names in ballpoint), and smiling... and always there. He's got a pristine minivan to take you in, and from the moment he says hello, all your luggage is in his hands.
He'll even stop off at the grocery store for you on the way.
I and my parents have used him three or four times now, and he's worth every penny. We even have a contract with him now through my work.
(if I never said it before Kristi, THANK YOU
)