http://www.collider.com/2010/02/09/darth-vaders-voice-before-voice-over/David Prowse did say all of Darth’s lines, muffled in a British accent in the big black mask and I’ve got some awesome video and audio to prove it.
via
http://www.scificool.com/hear-darth-vaders-voice-before-the-evil-treatments/
It's almost comical.