I think the score is very apt for the film. Let us be realistic: the underscore is supposed to express the feelings of the characters rather than be a complete pastiche of the music of the time. There was considerable overlap between the Classical and Romantic periods anyway. However, the main titles do allude to Beethoven, who would have written hundreds of pieces by the early 1800s, the period in which the book/film is set.
The diegetic music is very in keeping with the time; the pieces Marianne plays and sings are reminiscent of Romantic sonatas and parlour songs, and the text of the song is actually from a John Dowland lute song. The ball music is incidental really and has no importance in the story except to create an atmosphere, so Doyle has used late baroque/early classical characteristics which would have been best for dancing to.