Often smallish but a big cut above US convenience stores such as M&S/Sainsbury in the UK are pretty common in Europe generally. I still wouldn't be inclined to use them for lunch sandwiches in general but they have better than the slop you'll get at a US 7/11. I can't say I've really observed the dining area in general outside of some of the big food halls. (There are food markets with such but those exist in some big US cities as well.)
I think their lunch sandwiches are perfectly acceptable, I used to eat a Sainsbury's meal deal every day. Anecdotally I find the UK's prepackaged supermarket sandwiches are much nicer than than the equivalents on the continent.
