hmm...car rentals are always a problem for me (rate-wise, that is).
what i usually do is bug my sister to send me an entertainment coupon book, which has car rental coupons for all the major car companies, and i use that (ok, sometimes i just bug her to send me hers). i think there is a UK version (or at least london). the site is
www.entertainment.com, i think.
the coupons are for 5-10% off standard rates, then they have coupons like 'rent for a 3-day weekend, get 1 day free' or '$15 off one week' or a 2-car class upgrade. it's not too bad, and the bonus is you can check rates and book online.
again using
www.fodors.com/forums, i look for any car rental specials, or check out some of the smaller, regional companies and see what people have to say about them.
hope this helps some!