we use us rentacar and last year used expedia as they had 10% cashback through quidco
the car company we tend to use is Dollar, mainly because we have the dollar express card http://www.dollar.com/Express/LearnMore.aspx
We booked through Expedia this year, as they had the best price for what we wanted, and as a plus we also got Nectar points for the booking. I can't remember if you got a choice of car provider, but ours was with Dollar.
We always use Budget and is easy collection from the car park fasttrack easy, one time we used virgins dollar as we booked a fly drive and was a nightmare, wued for over 45 mins at the desk then to que in the carpark again and the car was rubbish not usual standard!
And also another April....were there in april too!