After the bombs dropped
You are looking at the strangest bomb damage repair job I've ever seen.
Very little of London carries any reminder of WW2, there are the occasional empty lot here and there, mostly concreted over and used for parking. Here, in the Mile End Road is one of the oddest remmanents. The East End was particularly heavily bombed, maybe that's why they just walled up each side, and put that funny bit in the middle.
I don't know what it was before the war, but from the appearance I would have said some sort of department store.