It might actually go the other way. The kings of France were not really strong in this time period. The kings of England being foreign monarchs allowed the French kings to focus their energies and vassals (once they got them under control a bit) to recovering Normandy, Anjou, Poitou, and Aquitaine. Most of the local landowners remained except in Normandy, but French nobles generally supported these wars because they could gain titles, plunder, and estates previously owned by men loyal to the English kings.
The English kings being basically culturally French and marrying with the French royal family led directly to the Hundred Year's War, too. While a bloody disaster in many ways, in the long term France emerged much stronger, more centralized, and more nationally unified than it was going into the war. Before and during the Hundred Year's War the French monarchy was struggling to maintain control and defeat the English. Afterward the English were absolutely a second rate power compared to France and Spain. Hence there was nobody around to help Brittany when France diplomatically and militarily pressured it to become part of France by marriage.
If Harold Godwinson wins the Battle of Hastings, then the powerful Norman dukes and the warlike Norman people remain in Normandy defying and occasionally warring with the French kings. They will likely focus energies on conflicts closer to home and this will be a detriment to the French kings. Brittany is as strong as any dukedom in France, and is certainly independent. The kings of France at this time are weaker personally than half or more of the dukes and counts, maybe in this time line Brittany remains independent and with close ties to the princes of Wales, kings of Anglaland, and kings of Scotland.