And did those feet in ancient time.
Walk upon Englands mountains green:
And was the holy Lamb of God,
On Englands pleasant pastures seen!
And did the Countenance Divine,
Shine forth upon our clouded hills?
And was Jerusalem builded here,
Among these dark Satanic Mills?
Since Blake's poem is clearly invoking the legend that Joseph of Arimathea came to Britain -- indeed even brought a young Jesus with him on an earlier visit -- the popular reading of the "dark Satanic Mills" as being the factories of the early Industrial Revolution does not make a great deal of sense.
The brooding monuments of Britain's pagan past make more historical sense, particularly as there is even sketch evidence from Blake's papers.
Oxford and Cambridge is another possibility.