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Having looked at a few google map street view snaps, I think it's almost impossible to have that kind of skyline pop up as big as that one right behind some woodland / canal area. I think there's simply no cluster of buildings as tall as that where such a view is possible, so you're probably right, it's a composite. Which means none of the scene has to be real. @chez_jay has pointed out a pretty good match for the style of the footbridge, the overpass is a bit generic in this picture and therefore more difficult to identify...
Wow the series Gangs of London is graphic.
Does anybody know if the location in the scene below is actually in London or just a composite for drama?