You get occasional birds on the reservoirs around london which will no doubt be on the rare bird services and then large numbers of Red-throated Divers winter in the outer Thames Estuary (in the thousands i believe).
Good place for Great Northern Divers (and sometimes Red- and Black-throateds) is Rolls Farm overlooking the Blackwater Estuary near Tollesbury in Essex. If you're there over high tide, you should also see Slavonian Grebes, with a decent chance of both scoter and Eider, and Shag.
Site is shown >here<; park sensibly on hard standing by the barn on the left before the road becomes private, and then walk down to the seawall.
It depends where you are in London. If west Staines is probably your best chance, its not the most pleasant of locations but there is always a lot about to pick through. Not sure there is general access at Queen Mother (anyone know out of interest?).
The Essex site mentioned above sounds more fun!
Old Hall Marshes, also in Tollesbury is meant to be good for Divers and seabirds like Eider and Common Scoter. I need to make the effort to get down there before Winter's out, but it's difficult sans car.