The best panoramic photos of London come from Tower Bridge - you'll capture not only the city itself, but also the Thames.
The most beautiful photos of London from the top can be taken from the Ferris wheel, located on the banks of the Thames. The attraction is called London Eye, one revolution of the wheel makes in 30 minutes. By the way, the "eye" works until the evening, so you can make spectacular photos of night London. Another popular object for photos of London at night is the Parliamentary complex.
Don't forget about photos of London streets - the most recognizable, "postcard" landscapes are obtained on Piccadilly Street or Baker Street (the main thing is that the name of the street on the house should be readable!).