This is so silly. Both of these hymns are in the Church of England's hymnal (Hymns Ancient and Modern). Furthermore, Blake's poem "Jerusalem" was actually written in support of the British labor movement suffering in the "dark satanic mills." The second verse is a call to action to build a new "Jerusalem" (a more godly and merciful place) in England.
Blake was a British Israelite who believed that Britons were one of the 12 Lost Tribes of Israel.
The hymn is based on a tradition that Joseph of Arimathea brought Jesus to england as a young boy when the former was in the business of importing tin. Not sure where the tradition comes from.