The script sucks and that is a primary issue, but I also feel Judi Dench's voice and inflections are a bit too "sharp" for this ride, though that is my personal opinion. I'm otherwise a big fan of her as an actress and don't dislike her voice at all outside of this ride. I just prefer someone with a more soothing voice as narrator for this ride.
Jeremy Irons is my favorite SSE voice largely due to this, it's soothing and hypnotic. I personally hope the future narrators can also pull off a similar tone of voice. British accents are nice, but not a requirement. The script and soothing tone of voice are more important IMO.
Here are some examples that I think could pull it off- Sigourney Weaver, Kate Mulgrew, Gillian Anderson, Liam Neeson, Cate Blanchett. In other ideas, James Earl Jones can be a surprisingly soothing and hypnotic narrator when not using a variation of his Darth Vader voice (though some might be against him as a choice due to the Star Wars connection). Morgan Freeman is another great voice, though he currently narrates HoP (not that he couldn't do multiple attractions). It's a shame Carl Sagan is no longer alive, he'd be an awesome choice too.