We've generally had good results booking cars through Opodo.
One piece of advice, always put your flight number on the booking
information. Some years ago, our flight into Stansted was delayed by a
few hours and by the time we landed, our reservation was cancelled as a
no-show and the car was gone. The were no cars to be had at any of the
other companies. We wound up taking the bus to Heathrow and rented a
car there.
T.
You could alway have a look at:-
www.holidayautos.co.uk
I always find that I get the best price/service from
http://www.enterprise.co.uk/car_rental/home.do
though I don't ever need to rent from a (UK) airport.
tim