This is a cool place to take pictures from: The hill at the center of the Greenwich Park, right by the Royal Observatory. From there, you get a wide view over the eastern part of London, from Saint Paul to the O2. The view is especially conductive to spectacular pictures, especially at dusk, when the sky turns a deep blue and the city lights up.
Keep an eye for the green laser that shoots out from the Observatory and goes across the river, marking the line of the Greeenwich Meridian. Bring a good tripod, as you will need to keep the shutter open for a while to capture the evening light at low ISO. A tripod will also come in handy if you plan to do a panorama, and who could resist with such a view? A polarizer might also help to make the laser beam more evident.
If the light is good, you will probably have to share the place with other photographers. I counted six tripods there last friday, including mine, without mentioning the numerous tourists with point-and-shoots. Don’t be worried, though, there’s plenty of room for everyone. When it starts getting really dark, the park’s warden will warn you that the park is about to close, so it’s time to pack up the tripod and head to a pub. Hopefully you will have made some friends with the other photographers and will want to share some beers and stories.