If you are referring to London then you have only seen what the tourist sees. All cities have their run down areas and Elephant and Castle in South London has to be the biggest dump I have ever had the misfortune of to step foot in.
Bristol, the other big Southern City, is very attractive but also has its run down estates and what not.
Well it depends what you look for in a palce to live. I would happily sacridice aesthetic surroundings and luxury living conditions for a city with atmosphere and a tangible culture of enjoyment. I live almsost perfectly between London and Milton keynes, I go to London on a weekly basis and rarely step foot in MK even though its shopping centre is superb as its cinema and all infrastructure.