To the question...
And was Jerusalem builded here?
I reply... Jerusalem Ln, yes, and Jerusalem farm too!
I think Blake is pondering the medieval myth that Jesus had visited England with Joseph of Arimathea and if the new Jerusalem from the Book of Revelations was ever created here, but destroyed by the Industrial Revolution (Satanic Mills).
Revelation 21:2
"And I saw the holy city, new Jerusalem, coming down out of heaven from God"
Blake wasn't taking these myths literally, but embedding subliminal political messages in his poem, (many) other interpretations are available.
I quite like some of Blake's etchings too.