Which actress played Viola in the movie Shakespeare in Love?

Lady Viola goes to America with her new husband. Shakespeare writes the play Twelfth Night about her.

Although the plot of Shakespeare in Love is about Shakespeare writing Romeo and Juliet, and it mirrors Romeo and Juliet in many ways, it is a very different story from Romeo and Juliet. There is no "Lady Viola" in Romeo and Juliet; she is a fictional creation of the writers of the screenplay.