Via mathcathy, an interactive map of london boroughs showing the most common surname in each. If you look at the most common, it's basically a sea of smith, but the default is to show something like 8th, where you can see an imperfect but striking map of ethnic makeup.
Website:
http://names.mappinglondon.co.uk/Blog post:
http://spatialanalysis.co.uk/2011/01/mapping-londons-surnames/