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: It appears that she somehow fell in and she was struggling to get out because Cruella was there and because the cake was about to be sliced to be baked.