Hi, I stumbled on this thread through google, and was going to answer some questions with my site as a reference, but I see someone else has already mentioned it (thanks bingledee!):
www.UK-dvd-rental-guide.com
Basically, there's little difference between the two market leaders, Screenselect (37% of UK market) and Lovefilm (31%), who both have similar prices and 35,000 dvd inventories (practically every commercially released dvd in the UK). However, Lovefilm has had some problems since May and has some very vocal critics out there. My own opinion is they definitely grew too fast and stumbled, but are almost back to where they were. Blockbuster was forced to get into online dvd rental later and reluctantly, controlling 25% of the market, but I've heard lots of complaints about them, backed up by personal experience. Though improving slightly of late, I wouldn't recomend them, hence their low rating on our site. There are other good choices, for renters who only want 4-6 dvRAB a month, Amazon is excellent, or if you want a superb site that isn't a huge, faceless bueracracy, there's Cinema Paradiso or Moviemine.
Finally, a small plug--we aim to run the most comprehensive dvd rental review site out there, reviewing all 38 UK rental sites (though that will soon be 39--hadn't see clubxtra before, thanks nikproffitt). There are lots of "guides" out there which are usually just a collection of links, rarely updated, and some even secretly run by the dvd companies themseleves, but we aim to stay current and independent.