I was in whsmith in Bury St Edmunds today and they had a box of clearance dvds and cdroms by the counter at very cheap prices. These will obviously vary from store to store but were all reduced below half price mostly because they were sets and the outer case was missing.
Titles in bury included season 6 of 24 reduced from £40 down to £13 (the outer box was missing but contents intact) which just gives an example of how much they had reduced things!
Probably worth checking your local store if your passing - you never know what you might find!
Also if you buy anything instore - I was given a voucher to get £5 off the golden compass dvd when spending £5 on stationery or books next week, and a £5 voucher valid on any book over £10 (also valid next week).