WHSmith has a few £9 Blu-Rays for half price. Instore at Leicester Fosse Park, not sure about everywhere else...
Forgetting Sarah Marshall - £4.50
The Eye - £4.50
Speed Racer - £4.50
Sex and the City - £4.50
Starship Troopers 3 - £4.50
Saw IV - £6
Spiderman 3 - £7.50
There are a bunch of others but can't remember them for the life of me...