Why let the truth ruin a perfectly good story....
The film is a British perspective.... the truth for the French was much grimmer. The French side of the story deserves its own film. As you mentioned the battle for Lille was heroic in perspective. The rearguard wasn't wholly French though, the British I Corp's assisted until they were evacuated. The film does play a little bit with time, depicting the evacuation as one instant.... not really the 8 days of action.
As the British forces were evacuated the French held the line as the I Corps pulled out on the 31st, the French holding the line allowed for the majority of the French to be evacuated. The thing is for more than half of the French forces that were evacuated it was a brief respite. They were quickly resettled, reorganised and redeployed to Normandy and Brittany, where most were killed or captured before France's final capitulation.... Not quite the warm glow of a reinvigorated Tommy sat on a train eating an apple, drinking a beer, reciting the words of a famous Churchill speech.
But as I said more a story worthy of its own depiction.