Is it possible? Kind of sort of, maybe, not realy, but technically yes.Is such a union possible if it is what he the effects of this Union.?
It would be quite the same as merging Google and Apple.
Massive destruction of value (1 + 1 = 1,2 maybe) since Britain and the United Provinces were competitors on many of the market segments where they had strong positions. Remembrer they west to war against one another several times.
Is it possible? Kind of sort of, maybe, not realy, but technically yes.
There was the time the Dutch rebels offered the rule of the Netherlands to Queen Elisabeth, which she refused, because she ddn't want to get into conflict with Spain.
There was the time when Cromwell offered a union with the Dutch Republic, which the Dutch refused, because they considered themselves better of without being ruled from London.
And lastly there was the kinda sorta personal union between the Netherlands and England.
All of those could technically work, but aren't likely to. Besides those? I don't think so.
What would be the effects? Hard to say. Depends on when and why and how.
What about having the Bloody Mary Tudor married to the count of Cleves.
Cleves is a German ministate, not any kind of county with any ties or rights over the Netherlands. It might just mean that the English have another Hannover on their hands, although I am not sure how the lines of succession would work out in such a situation.
And the Lutheran character of Cleves would make any match with Mary Tudor a difficult thing to manage.
Henry VIII wanted this marriage to happen to Mary but did not happen because Mary does not want to consent to this marriage, Henry VIII could force his daughter to marry him, the question is how?
If Henry really really wanted it, it would have happened. It did not end up happening because it was not nearly that important. Henry was convinced he was going to have a healthy, male Protestant heir. Mary was the spare.
The point of the matter is that whether Mary Tudor ends up marrying into some small German county or not, it will bring the English no closer into having any sort of union with the Dutch.
Why not go a bit perverse with this - if the major issue behind an Anglo-Dutch union is exposure to mainland attacks, why not have the following
1) The Hundred Years war is won by Henry V, who has two sons. He makes the younger the Duke of "Francien", essentially the Royal Enforcer, and his eldest King of England. Eventually the War of the Roses in England is ended by the Duke of Francien coming across and crushing the Yorkists, and becoming King of England.
2) After the Hundred Years war and Roses are dealt with, then at some point the House of Lancaster/Royal House of Anglo-France, has the opportunity to take the throne of the Netherlands. Having their wealth, and traders would help the Empire, and France can EASILY provide the manpower to defend it, and as such is accepted.
Boom - Anglo-Dutch union via Anglo-French union.
Can we really say that William "ruled" the Netherlands? Didn't the Staaten General have a lot of say in everything the Dutch did, and could even strongarm him?
Also, if memory serves, he wasn't Stadtholder of the whole Netherlands, but mostly of Holland and usually Utrecht and Zeeland. But Guelders and Friesland had other people as Stadholders. It seems to me that at any point the Dutch could break the union, even in the case of a son for William and Mary, by either electing another individual or going the Stadtholderless route which they did historically.
Franco-Dutch union, more like. This is just mega-France.