MVC can get some good bargains, but also try Woolworths and WH Smith. My local Smith's had all the boxed sets of Star Trek:TNG at about £30 each [note to Americans - this is cheap for the UK]. Woolies had various half price discs (plus some nice Playstation and XBox stuff) - nothing I personally wanted, but it looked a decent selection. However, I think it's worth noting that there can be a huge variation between individual stores.
You might find the 'Bargain Buckets' section at the following site useful:
http://www.dvd.reviewer.co.uk/
