This has an effect on the middle classes, especially social climbers and they too will adopt this prestige language and become de facto bilingual. This is something, which was also seen IOTL in the various countries referred to in this thread.
Assuming roughly similar social development in Europe, I would say that in the 17th and 18th the rising middle class will try to emulate the upperclasses, including speaking their language, but once in the 19th and early 20th century the people will become richer and the middle class grows with people from the lower classes, I think you'll see more and more middle class people speaking the local languages until in the middle 20th century the upper classes will speak the local language themselves. This will be the point when Franco-Britain could split. If the Francophone elite will enforce their language on England, you will see trouble arising and a split. If they are willing to accept a bilingual and cultural country, the union can continue, at least for a while.