London has a very high ratio of extremely nice houses on a road opposite council houses, or former council houses. There can often be a very large mix of housing in one area.
FWIW ancient Rome was also like this, and for example in Pompeii you can find extremely fancy houses with frescoed dining rooms right next door to single room hovels. They didn't have subways or mobile phones though.
It seems to me to be quite the feature ... well, everywhere except the USA. Certainly all over Europe, one finds this mix. In the USA, you generally only find cheap/low quality/small housing stock adjacent to expensive/high quality/large housing stock where there's some municipal or other border, and the two just can't avoid being where they are.