Around here, one of the best places for DVDs seems to be Morrisons, and there can be some good prices on VHS too, if you`re so inclined.
Currently, their "chart" rack has Fellowship of the Ring, extended version, at £19.99 and Zulu at £12.99 and generally lower prices than other stores. Most of the ordinary DVDs are currently at £9.99, or two for £15 (Men in Black, Saving Private Ryan, The Mummy), and I picked Dogma out of a bargain bin for £5.99 (similar mix to bins in other supermarkets, but lower price).
The local Tesco has been running a bargain bin for several weeks, but at higher prices. Kwiksave had a bit of a price cut after Christmas. On the whole, Morrisons seems worth checking first.