Oh, are you referring to the use of the apostrophe? In this case, I would say it's appropriate, since I guess I mean to imply "Sainsbury's supermarket" (even though, I know the name is Sainsburys.) I don't think I've ever heard anyone misuse "Asda" or "Tesco". Granted, I'm not living there full time yet, so my interactions have been limited!
FTR, Sainsburys is my favorite UK supermarket!! (Even though DF's last name is Morrison! 
)
Just the "s" slapped on to everything (not necessarily the apostrophe), both spoken and written... going to Asda's, got it at Tesco's, saw a book at WH Smith's, bought a dress at M&S's. I hear it a
lot. (That being said, I apply the teeth-gritting equally to the US; Barnes & Noble's, for example.) fwiw, Sainsbury's is the correct thing to say anyway.
I don't often shop at Sainsbury's but I don't know, I might start.... I feel like Tesco are slipping, with things like tidiness, organization of aisles, all products having a price label on the shelf. Maybe it's just me.
Lidl or Aldi we call "Didldidi" (from "That Mitchell and Webb Look").
We do the same thing! I now have a hankering for bacon octopus shapes covered in glitter....
