The archbishop of Canterbury is the spiritual head of the Anglican Communion, of which the Episcopal Church is part, and is the most senior bishop of the Church of England. He has no authority over other national Anglican/Episcopal churches but does hold a place of honor among them.
Capitalize the title when it precedes the holder’s first and last name; on second reference use only the person’s last name. Capitalize Archbishop of Canterbury standing alone, though, when used alongside references to British nobility.
The archbishop of Canterbury is also referred to by the honorific the Most Rev., as in the Most Rev. Justin Welby, archbishop of Canterbury, but it is sufficient to refer to him as Archbishop of Canterbury Justin Welby.