Yank here. I’ve never visited London, but when seeing “28 Days Later” I noticed a hell of a lot of cranes dotting the skyline. I noticed the same thing in “Love, Actually”. Now, SF and Philly are my two main frames of big-city references, and I never really noticed dozens of cranes.
This might be the one of the stupidest questions ever, but why in the hell does London have cranes dominating the skyline?
Because there’s a lot of building going on! I think London’s skyline is still evolving more than a lot of American cities. I went to see Love Actually last night and I noticed that the “Gherkin” (or 30 St Mary Axe, to give it its more prosaic name - it’s the sort of, well, gherkin shaped, rounded, ovoid thing that you could see behind St Paul’s) was very prominent in a lot of shots - at the time it was only half built, but is now nearly finished. What will be Europe’s tallest building, the London Bridge Tower, was granted planning permission last week.
Other big cranes at the moment are those constructing a new office complex near the Tower of London, replacing a truly hideous 1960s building called Europe House that I had the misfortune to walk past every day on the way to work, before it was demolished.
But if you really want to see a lot of cranes, go to Berlin!
In the UK cranes are considered to be the peak of architectural glory. 
Actually, there is an old saying that goes something along the lines that if you stand on Tower Bridge and see more than seven cranes to the right/left?? of St Paul’s Cathedral, then times are prosperous. That is obviously a very old saying as there are usually many more than that now. I have no cite for this…sorry.
Bear in mind that London does not have such a tall skyline (like many US cities). Therefore, one can typically see further and cranes would appear to be more widespread than they would in somewhere like NYC, where they would be shielded by other, taller buildings.