EDIT: Make sure the items sold are sold by Amazon and not a third party. Some items default to a third party seller and you need to specifically choose to buy through Amazon.
There is currently a price error on Amazon which doubles up on discounts on a select range of Blu-Rays and DVDs. The DVD or Blu-Ray needs to be in both the 2 for £10 offer and the 3 for £20 offer to apply.
I bought the following for the sum of £18.29 (free delivery because it was over £20 before the discount):
12 Years a Slave Blu-Ray
American Beauty BR
Hot Tub Time Machine 2 BR
An American Werewolf in London BR
Watership Down BR
It appears adding different items has a different effect on the discount. I have seen it lower than the £18.29 I got those for. I believe it can go under £17 for six items.
Other Blu-Ray titles that appear to be in both offers include amongst others:
Guns of Navarone
The Big Lebowski
The Mummy Returns
The Truman Show
Hellboy 2: The Golden Army
Children of Men
From Here to Eternity
Escape From Alcatraz
Anchorman: The Legend of Ron Burgundy
Battlestar Galactica: The Plan
About A Boy