Aren't Wallonia and French Flanders(Calais) francophone?
Welll, sort of. French Flanders as in Dunkirk used to speak Dutch,I believe up to the late 19th century. There are still people who speak a Dutch dialiect in that region (West-Flemish), although it probably will only take a couple of decades before they are all gone. Even in Calais people spoke Dutch during the Middleages.
Anyway, there are various ways for the Netherlands and England to form one country. The most often used POD is stadholder Willem III, who became king of England (as Korporal Nooij said Flocc wrote a pretty cool timeline about it), although I must admit I don't think it will hold. The Dutch were too independent minded to be ruled from England. Still it could work.
There are also a couple of other options. Elizabeth was offered the Dutch provinces if England would support the Dutch struggle against the Spanish during the eighty year revolt. Elizabeth refused though, fearing that the Spanish wouldn't like it (which they didn't, it was one of the reasons for the Armada).
Another possibility is Cromwell, who wished to create an Anglo-Dutch union. The Dutch refused though, but they could have accepted, certainly if the French decided to attack the Netherlands, like they did in 1672.
There are other possibilities. A Anglo-Dutch personal union was possible in the early 19th century. The future Dutch king Willem II was engaged to the British crown princess. I believe Nek was writing a timeline about it, before he abandoned it.
And if we go back to the middleages, everything is possible, although someone somehow gaining all Dutch territory is reasonably unlikely (certainly if he isn't focused on the Netherlands, like a English king who would be focussed on England).