Was in a large Tesco Extra today and noticed they had a large display of DVDs, CDs etc with significant reductions. It appears this is both instore and online.
Batman Begins £3
8 Mile £3 (£4 online for some reason)
Prices online and instore do seem to vary, so it's maybe worth visiting your local store to check out the deals.
Forgot to add, there are many other titles, the above is only a small amount of the selection!