Far too late for Autumn colours. You might get some snow, which would be something in London. Unlikely though.
I suppose there is the usual stuff: Houses of Parliament, South Bank. the Gherkin, river Thames and bridges esp. that suspension one, the Tate Modern and wobbly bridge, Buck Palace, China Town, Trafalgar Square, Thames Barrage, Natural History Museum, Albert Memorial and Hyde Park, Rotten Row and horse guards, Golders Green and orthodox jews (in religious garb), various train stations but probably no tripods allowed, London Underground (but no tripods and no flash allowed), Greenwich Museum and various ships (HMS Belfast), Windsor Castle, and so on.
Sadly places like St. Pauls Cathedral ban photography inside.