Blake's poem "London" is the antithesis of his earlier poems in "Songs of Innocence." In the earlier poems, innocent children frolic,nature is in bloom, and people are happy and loving. It is a time parallel to the time before the Fall. But, in "London" which Blake chooses to attack specifically the corruption in England's government and church, there is no allusion to a natural world except the Thames River, which, unnaturally, has been "charter'd"; that is, owned and bound by British law.
Thus, London represents the evils of English society as the capital of England and the center of its culture. The strength of Blake's poem lies in its ironic contrasts. The chimney sweep's cry is an affront to the Christianity that the Church of England promulgates and the soldier who fights to preserve the monarchy sheds his blood for only the palace walls:
How the chimney sweeper's cry
Every blackening church appals
And the hapless soldier's sigh
Runs in blood down palace walls.
Blake also decries the institutions of English rule, centered in London. He writes that
In every voice, in every ban
The mind-forged manacles I hear.
With ironic use of "ban" for marriage that binds together people who do not love one another, Blake reviles the one sacrament that should offer hope: marriage. But the unhappy husband goes to the harlot, who in turn gives him syphlis that he passes on to his wife, causing "the new-born infant's tear" as it is blinded.
By walking specifically through the streets of London, the capital city of England, Blake uses imagery and irony to point to the egregious conditions of his English government and church.
In my opinion, London in this poem is really just a stand in or a representation for all cities in the world and for all of civilization. The poem is not really one whose ideas are specific to London or to England -- they are universal.
The poem is talking about the ways in which people have become degraded by living in cities -- by living in "civilization." They have had so many aspects of their humanity taken away or watered down by this. This is clearly something that is not unique to London.
So I would say that London represents all of modern human civilization and especially the urban version of that civilization.
London represents a fallen world. The poem purports to be based on the personal observation of the speaker, the “I” who has observed these abuses and horrors. Every person the speaker observes has been blighted or plagued, and the midnight streets heighten the darkness, misery, and danger. The speaker mentions cries both of adults and children, the cry of the Chimney Sweep, the sigh of the Soldier, the shrill curses of young prostitutes, and public pronouncements. These are sounds of sorrow, rage, poverty, and debasement; they symbolize the political degradation of human beings. Having made these observations, the speaker is qualified to speak from experience. The inclusion of the poem in "Songs of Experience" seems reasonable.