Double exposure or multi exposure is a technique in which the camera shutter is opened more than once to expose the film multiple times, usually to different images.
This was taken from a restaurant high on top of the Shard. Britains tallest building and 7th in Europe, the architect, Renzo Piano, describes it in his vision as the vertical city. Being atop an iconic building with one of the greatest cities at your feet, stretching beyond where the eyes can see, is a magical experience.
I have innumerable pictures from different view points but this one clicking the same shot at different length in the same frame seemed the most dramatic and match how much the eye was trying to take in.