Many parts of London were brand spanking new,mainly the newly built "West End",where fashionable persons had chosen to live.
However,step a short walk away from this to the seven dials area(near Covent Garden) and a whole new and dangerously run down area would appear.
There were many of these areas in London of Dickens time,where the lowest of the low of society lived,and just about existed.These properties were owned by absentee landlords (read Little Dorrit)and were nothing more than downright slums.
They were known as rookeries (after the large gatherings of rooks in trees,
Fagin lived with his gang,and Bill Sikes and Nacy lived in such a rookery.
Children abandoned by their Mothers ended up here,in fact most of Fagin's gang were such children.
Dickens,being the social reformer that he was featured more of the rookeries etc in his books that the upper class areas,However,he DID bring new upper class characters (and their movements up and down the social ladder, into his books,but as one of his main aims was for social justice the poor obviously came first.
So to get back(eventually LOL) to your question.
Many of the buildings in London WERE new,but an even bigger percentage were not,and as such prove more tempting to film/tv directors.
Also don't forget that London of the time was a dirty, smokey.smelly place,and this is probably reflected (hopefully) in any production that tries to be at least a small bit genuine.
Ithink it would horrify most 21stC peeople to go back to London of Dickens time!
If you want any more information just ask.
I belong to the Dickens Fellowship,so can bore for England about Dickens.LOL