Now hold on just a second. "London" is from Blake's Songs of Experience, not Songs of Innocence, right? Yep—which is why innocence is a major theme. Huh? Well, we don't mean innocence so much as the ways in which people have their innocence taken away (mostly infants and children). This poem is also interested in guilt, too. The church and the government, for example, should be innocent of wrongdoing but, strangely, are guilty of taking innocence away from children and infants. Sheesh.
Innocence? What innocence? In the London described in "London," there is no such thing as innocence anymore.
This poem cannot describe innocence because the speaker himself is not innocent. He's a dude that clearly has some experience of the world.