Have you booked a car yet? which company?
If not USrentacar do good prices. Also rented thru Airnet for years with no problems but there are many other companies.
Never arrive at the airport without prepaying in UK. It can be about double the price we prepay here. My Brother did it in Vegas last year and cost him a fortune.
If its Dollar you are using then Dollar express is free to everyone, Budgets scheme is supposed to be available after you have rented 3 times but the link I posted lets you register on 1st rental, if it allows you, like I said some have problems with it.