To and from Cambridge I (highly) recommend Cambridge Connections, who I use regularly:
http://www.cambridgeconnections.co.uk
In the executive transfer market I've also used Voyager Executive Cars in the past, but they cranked their prices up rather high a year or two back.
Panther will probably do the job, but I'm not to keen on some of their drivers, my preferred local taxi company is A1 Cabco - however either of these may well not provide as comfortable a ride as Cambridge Connections. Much as I like cabco for local taxis, I would go with C C for an airport transfer.
C C use all Merc, Audi, large Volvo, Jag etc. A taxi company might send you a town 6-seater with very hard seats for 2 hours! C C charge about 100 quid for the trip one way, and will wait for you by arrivals in LHR and/or call you for an update if there are delays (of traffic or aircraft).