I find it varies all over the place. Where I live we shop at Sainsbury's in Dunstable for our groceries. Which is the closest to where we live. A Mid size supermarket. Has an Okay range nothing special.
In the other direction Harpenden smaller Sainsbury's with a much better and varied selection. The difference being Harpenden is very upmarket and housing is much more expensive. Hence the selection in the store is better.
Go in to Waitrose in Harpenden. I have never seen anything unusual or that I don't have already, very dissappoiting.
Yet go into Asda or Tesco which seems to appeal more to the masses they always have some really good offers with some bottles being discounted by up to a third.
The moral of the story for me is to shop around all the time. Thats why I love it when someone flags up an offer with the supermarket deals thread.