Indeed, Gaucelin the Cruel conquered England in 1359 or so*, uniting them under the Van Vlaanderen dynasty. They only split up in 1443**, because England had primogeniture and France had elective succession, at which point they were both still Van Vlaanderen and I presume they both still had Frankish monarchs.
Not sure why they'd stick with French culture when they're under the Von Chelyabinsks, though.
*"Reaching the age of majority in 1356, Gaucelin was determined to make an immediate impression. Egged on by his ambitious Marshal, Duke Wilhelm II, he took advantage of the insurrection in England by declaring war to capture the crowns of England and Ireland that had been worn by his grandfather.
While he was campaigning in England, the new Duke of Gascogne, Emmo II, followed in his father’s footsteps and declared independence. Unwilling to return home, Gaucelin persevered in England and three years later led his troops into Westminster, forcing Mark capitulated."
**"But the Kingdoms of France, England and Ireland were not destined to stay united. Early in Richard’s father’s reign, when King Gaucelin had been a child, he had agreed to elective succession for the Kingdom of France. When Gaucelin had died, there had been no controversy. However, Richard’s eldest son had died in 1430. While he had a son who ruled as Duke Ontfroy of Kent thereafter, the French nobles knew little of him and the reports they had from across the channel did not impress them. Upon Richard’s death in 1443, Ontfroy became King of England and Ireland, but not of France. Instead, the nobles chose Richard’s son, Duke Guillaume of Anjou. Guillaume had been crippled in war, but he was still handsome and popular, with a reputation for kindness and diligence that contrasted with Ontfroy’s rumored nature."