The 'dark satanic mills' are NOT churches, but factories. They were destroying a way of life, driving people into horrific poverty and were foul blots on the landscape. This is a cry to arms, to create a better world. Blake was a radical, then and now.
Christ visiting England is a misreading I think, it's hard to explain, but feels more like he is talking of a lost ideal, to which we must return.
How using England makes it imperialist or jingoistic, I still haven't figured out.