On a shot in the dark, I googled something I saw on a TV show (Anthony Bourdain, I think) a long time ago, and BINGO!
The small structures in front of the London facades are Cabmen's Shelters.
Wikipedia article:
http://en.wikipedia.org/wiki/Cabmen's_Shelter_Fund
Perhaps they'll serve sandwiches and snacks for guests passing through and not entering Diagon Alley.
EDIT: I found the episode I remembered. The cab shelter part starts at about the 40 minute mark.
[video=dailymotion;xo2mr7]http://www.dailymotion.com/video/xo2mr7_the-layover-s01e09-london_travel[/video]