There may be hundreds of different kinds of boot scrapers out there but, so far, I can only identify six different ways of fixing them.
Reading clockwise from top left:
Recessed; Step-mounted; Gate post; Free-standing; Railings-mounted flush; Railings mounted proud.
The free-standing and railings-mounted types seem to have been more favoured in the central London area. Islington, however, is littered with the gate-mounted types which seems a better idea to me, keeping the mud and mess further from the front door.
The two pics at bottom left show that many houses or establishments had two or more outside the door. Shown here is a row of houses on Bedford Square, each house having one each side of the door and, just around the corner in Bloomsbury Street near the British Museum, two side-by-side which have another matching pair sitting opposite them across the path.