Was in my local Asda last night, and they had two small clearance bits set up (yellow cardboard shelves). They had a number of DVDs and CDs (nothing remarkable, they had alot of their 3/4 CD compilations, e.g. "100 movie themes", etc), but mixed in with the DVDs were some blu-rays for only £3.50! I picked up "No Country For Old Men" (down from £15), but they also had "Donkey Punch" and some vampire film Lucy Liu (think it was called "Rise: Blood Hunter").
I never bothered with those other 2, but one of the staff told me they had bought Saw III for £3 on blu-ray a couple of nights before, and that they had a bunch of other blu-rays in the back. However, he couldn't tell me what ones would be getting sold off for cheap, and when they would be. He just advised me to phone/check every day. Apparently, they had gotten them off a company who went bust (who also used to stock for Woolies, according to this guy), and they will just be selling them off over time.
NOTE: this seems to be instore only, and while stocks last. Blu-rays/ DVDs/ CDs will probably vary between stores.