I have been across the Channel a few times; but always by plane. So I am uncertain as to how long it takes for a ship to make the crossing; still, can it possibly be as long as this movie depicts??
With the Chunnel now in operation; do they still have these vessels??
I have crossed from England to France by ferry, and it only took a few hours, it certainly didn't require an overnight trip.
I went from Dover, and I know in the film it was to Portsmouth from Le Havre in France, but I am still puzzled as to why it was such a long trip. Perhaps ferry's travel slowly.
Portsmouth to Le Havre takes about eight hours thereabouts.
Also it tends to leave Portsmouth at about 23:00. So would be an overnight trip.
Hope that helps.