I was curious so I looked at the identifiable image[1] in the article in google maps[2] and it seems that only some of the crossings have these notes - notably the one way street crossing has look left/right on the respective sides of the road and the crossing with the island has them on both lanes, but the crossing over the two way street has nothing. I can't say I've ever actually noticed these (which may be because they're less common outside London or because I just don't pay close enough attention to the floor at my feet when walking).

[1] https://cdn-images.postach.io/0bd25fcc-8ab1-40fe-8eef-bcafaa... [2] https://goo.gl/maps/LB8rj7FJNLw