Factual error: The train leaves for France from St Pancras station (clearly identified by The Shire pub), which is Northbound and has no connection to go back south for France.
Factual error: The puppies don't add enough flour for how thick the cake batter gets.
Factual error: In the scene where the train is leaving the station, a locomotive warning bell is heard. British trains do not have bells.
Answer: Because Jeff Daniels and Joely Richardson who portrayed Roger and Anita were busy filming other projects at the time and maybe they chose not to return. Or perhaps the writers couldn't find a way to fit them into the Story. As to how Dipstick got adopted by Chloe, maybe it's just a mystery we'll never know.