I once had a very bad experience with Forest Hill Cars. I booked in advance for two adults and a toddler and some luggage. I was told it would be £40. The driver arrived at 4am in the middle of winter and decided to argue with me about whether it was an 'estate job' or not. He had a great big people carrier, so it wasn't as if he couldn't take us (and we could easily have fit the luggage and people in a saloon car), but he insisted on a £20 (50%) extra to take us to Heathrow which the office backed him up for at the time. At 4am in the middle of winter with a toddler. I later complained to the office and got nowhere. Suffice to say, I haven't used them since despite them being five minutes walk from home.
Last time we went to Gatwick, we used the parking facilities there which was roughly £70 for a week. When travelling for work, I usually get the train as it's normally a change at East Croydon and then you're pretty much there.