Avoid GreenMotion at Gatwick
Being off airport, they know they can stick you with extra charges as you are unlikely to want to go back to the airport and use another car rental company. I had my reservation cancelled because I was four hours late picking up the car, even though my unrefundable deposit more than covered the first day's rental. Either I could waste another hour getting back to the airport, or I could pay three times as much as my original reservation. I asked why picking a car up late was a problem for them and they claimed that someone else had taken "my" car (unlikely, since you wouldn't go all the way there unless you had a booking) so I had to pay for an upgrade. Of course, they claimed there were no other cheap cars in the lot (out of the 40+ there) so I had to take an expensive one.