Well, I highly doubt Sondheim had any say in the matter. This is not a revival of one of his shows, this is a movie and Sondheim probably only participated in the casting as an adviser. Perhaps Burton wanted Sondheim's sign of approval but legally-wise, I doubt Sondheim could have done anything to keep Carter from being cast.
Winslet was in the running when husband Sam Mendes (whom I probably would have preferred as director) was slated to direct and Collette auditioned according to rumors but allegedly Sondheim told Burton that Lovett should be older than Todd.
Of the actresses we know were rumored, considered or auditioned at one point or another (Bernadette Peters, Cyndi Lauper, Meryl Streep, Kate Winslet, Toni Collette, Nicole Kidman), Helena Boham Carter is the most lackluster of the bunch, IMO. However, let's not forget she is an Oscar-nominated actress so I'm sure she won't be a disaster in the movie.
"Some people can thrive and bloom living life in a living room, that's perfect for some people of one hundred and five. But I at least gotta try, when I think of all the sights that I gotta see, all the places I gotta play, all the things that I gotta be at"
