You usually serve yourself at a buffet, and it’s usually from one vendor. A canteen like this could have multiple vendors. A buffet is also usually “all you can eat” and priced to account for that.

I guess London doesn't have the equivalent of Golden Corral or Old Country Buffet, or a decent diner now that I think of it. I wonder if she moved to English equivalent of the burbs there would be something more inviting.