I daresay it's actually pretty passable when I'm just speaking. But narration has its own "accent," that straight non-regional thing. I can't do straight non-regional and British at the same time. British to me is all about the conversational lilt, which narration by definition can't have.
I actually used that movie maker site someone had posted here awhile back (
www.xtranormal.com) for reference, since I figured a computer-generated British accent was pretty close to what I was trying to hit.