Don't know if this is nationwide but in my local PC World / Currys Megastore in Bristol Cribbs Causeway they are selling off ALL of their DVDs for £4.97 each and if you've got a £5 voucher you can pick one up for free. I had a couple of spares so managed to get myself Where The Wild Things Are and The Men Who Stare At Goats for the pricely sum of zero English Pounds.
There is a LOT available, including recent films like Sherlock Holmes and Alice in Wonderland, even a few TV Series too, saw Entourage, Curb Your Enthusiam and Everybody Loves Raymond series' boxes there.
If you've for a spare voucher go check it out, if not then sign a voucher up to your Grans address or something and get one. Free DVDs!