That's pretty much the way it is with me, Bill. My speaking voice is pure SE England, almost cockney, but singing brings out more rounded vowel sounds. UK members will know that some of the comedy songs by Chaz & Dave require a cockney accent, but most other stuff really doesn't benefit from that.

I once read somewhere about Dusty Springfield, when asked about her vocal styling on that Memphis-recorded album which featured Son of A Preacher Man, said her accent was "mid-Atlantic". A very curious description.