http://www.dvd.reviewer.co.uk/forums/default.asp?Forum=222 here for checking about retailers opinion and on this forum at the dvd section, but beware no retailer is perfect, for example dvdsoon, I have never had any problems whatsoever with them, which I think goes for majority of customers here but there are those that got bad luck and bad customer service, and their opinions will be heard on both forums.
Also there seem to be risks of custom charges when ordering from playusa and cd-wow if you order over 18pounds.
Update! it seems that dvdsoon is also being targeted! They have to declare the full value on the sticker.
Now that USD is very weak, do have a check on www.dvdpacific.com as well, well good prices. Canadian www.dvdcellar is also extremely cheap now, at the same league as dvdsoon.
finally, don't forget to check the bargains department here.