Andrew, have you been in my house? I have a sloping floor in the room above.
Yes, the room above often also has a slope, usually towards the outside drain, this is not a feature but subsidence, usual causes are WWII bombing in the area which cracked the clay pipes, or dry summers that did the same thing (back in the dim and distant past when we had summers)
Also with those Victoria terrace houses, they got the good experienced brickies to build the fronts of the houses, whereas the rear of the houses was built with cheaper bricks by the apprentices, often with little supervision, hence the back of the houses is not the same quality as the front.