The suggestion about the picture rail having been removed is interesting but my gut feeling it's not that. This is however the kind of house that might have had a kind of plate display shelf in the hall downstairs. I don't think it's the era for a picture rail.

Where there was a plate shelf downstairs it was clearly attached on top of the plaster rather than having been attached to the brick.

It's so neat I think it was done as a feature - it's quite clever I guess and it's been done really well rather than looking like it was the done after anything was removed.

Just for the record the house has cavity walls.