Regarding his conversation with Stephen Fry... that's not why they call it Delta. Delta is a term referring to the terminus of a river that, due to silting, has split into multiple branching, forking rivulets. It's named after the Greek letter 'delta', which is shaped like a triangle, because it is shaped like a triangle, which is what happens when a river forks, forks again, and then reforks. Flooding is something that happens to rivers, but it's not delta. Delta blues is so named because it originated near the delta of the Mississippi and then, because the Mississippi was the major highway, spread up the river.
Is his voice naturally that calm and authoritative or did he work his ass off to make it that way?