Yeah, city stores tend not to stock the things that are in the bigger stores. Even our sort of full sized Sainsbury's would probably be seen as bare bones compared to what many of you used to shopping at bigger stores are used to seeing.
When I've shopped at Waitrose (never been to a full sized one) or Morrison's (nearest one to me, Camden Town which is a bit of a bus journey), I wasn't looking for buttermilk. The Sommerfield I've been to on the West End is slightly bigger than a convenience store, and I would be shocked to see it there. They literally had like two types of Cheddar on offer when I was there, which is almost Un-English.
ETA: When I say "city stores" I mean near central London. Lots of bigger stores seem to be available the further you get from the Square Mile.