score:25
Nothing happens at all.
This is essentially a question of two parts. Part one is unstated, but important, and it is the question of who is legitimate monarch.
First of all, legitimacy does not, as Tony Robinson claims, rest on blood. Legitimacy rests on being accepted as legitimate. This sounds like a tautology, and on some level it is, but on another level it is not. The rightful monarch is the one that is accepted as rightful monarch, and this is often stated in some act of law. In many cases that law is a list of rules on who is "the rightful" heir, but who is it take makes up that list? Right, whoever is currently in power. There is no "God given" correct succession. No-one has a right to be a King or Queen that exists outside the peoples acceptance of him/her as monarch.
The legitimate succession is therefore the one that is accepted as being legitimate.
In England, the rules of succession has been changed several times, most lately in 2013, mainly to make succession gender-neutral. England even was a commonwealth between 1649 and 1660, and there was no monarch at all. Now, British law is complicated, so exactly what laws are relevant and still valid is beyond me (apparently the Treason Act of 1351 is still valid, just as an example, and had to be modified when changing the succession in 2013), but Wikipedia claims that the relevant laws all say that the succession are legitimate descendants of Sophia of Hannover. So whether Edward IV was legitimate or not is irrelevant.
Saying that the Windsors are illegitimate is therefore incorrect. Had Edward IV not been accepted as legitimate somebody else would have been monarch today. This is the core truth in the TV program. But it does not make Queen Elizabeth illegitimate. Current British law says that the legitimate monarch is Elizabeth Alexandra Mary of Windsor, and she is also the one that is accepted as the monarch.
But what would happen if theoretically it is shown that the current monarch is not the lawful monarch?
Most likely, what would happen is that a law would be passed, making the monarch the lawful monarch. It is of course quite possible that the current monarch somehow is implied by the scandal himself/herself and is forced to abdicate. Say that it turns out that Queen Elizabeth is not the legitimate child of George VI, and that she has been active in keeping this a secret. Who then becomes monarch? Well, whoever has the support. Although in this hypothetical case the nearest legal heir to the throne really is Prince Richard, Duke of Gloucester, it is also quite possible that Prince Charles instead gets to be monarch. He might not be the legally correct one, but he is born and bred to be monarch, and giving him the title would be easiest, as otherwise pretty much the whole royal house needs to be changed. He also seems quite popular. This will likely also require an act of law, but since Prince Richard is unlikely to have a large amount of followers in the parliament sitting and waiting to make him King, it's a law that would be easily passed.
Or perhaps it would be the final nail in the coffin for the monarchy and Britain might be made into a commonwealth again.
So onto part 2 and the stated questions:
How would this affect a kingdom (i.e. UK) today?
Either a new law would be passed that makes the current monarch (or a descendant) the lawful monarch, or whoever is next in the line would become monarch, or the monarchy will be abolished.
Would the new head of state need to re-affirm age-old decisions and, if so, how far back would they have to go?
No. A monarch stepping down does not invalidate any laws or decisions made under that monarch.
Has this situation ever arisen in the past, how big was the gap (I'm tempted to call it the "Royal Hamming Distance"), and how was it dealt with?
Not that I'm aware of. In the past, if this situation would have arisen, at it possibly did with Edward IV, it would simply be hushed up and ignored. Today with democracy and mass media this would be much harder to achieve, but should it be discovered somehow that somewhere in the line between Sophia of Hannover and Elizabeth II there is an illegitimate child, nothing would be done about it. The monarchs since then were all accepted as legitimate, and therefore they are legitimate.
(As a side note, Richard III was able to get Edwards IV's marriage to Elizabeth Woodville declared invalid after Edwards IV's death, which meant that Richard III became the new king, instead of Edwards IV's son Edward. Of course this just happened because Richard III had the practical power and support in the parliament at that day, and he also had Edwards IV's young sons killed. So if Tony Robinson want to find the actual "real king" of England he should look amongst Catherine of York's descendants, but I think that line might have died out, making for a boring TV program. But it's an example of what might happen if you decide that someone is not legitimate.)
Upvote:3
If you wonder if any historical discoveries or speculations about the legitimate or otherwise birth of medieval persons can affect the claim to the throne of the present British monarch, the answer is no. Her claim to the throne begins with the events of 1688, and no prior historical events can make it stronger or weaker. If the world began in about the year 1550 it would not change her claim.
After the childless King William III and Queen Mary II usurped the thrones of England, Scotland, Ireland and France in 1688-89, the English Parliament passed several laws restricting the future succession to the throne to the nearest Protestant descendant of the Stuart Dynasty. They included the Bill of Rights of 1689 and the Act of Settlement of 1701. When England and Scotland united to form Great Britain in 1707 the Protestant succession was reaffirmed. Thus in 1714 George, Elector of Hanover, became King George I of Great Britain, Ireland and France. In 1801 Great Britain and Ireland united to become the United Kingdom.
Since 1714 every monarch of Great Britain or the United Kingdom has been the legal and rightful successor of George I, who was the nearest Protestant heir to the Stuart Dynasty as mandated by the Bill of Rights, the Act of Settlement, and other laws.
Unless, of course, some rumored royal secret goings on were correct. King George IV, for example, was a bigamist, and it is unclear which, if either, of his marriages was legally valid. And Mrs. Fitzherbert, his secret wife, never answered the question of if she had any children with George IV.
Queen Victoria, the niece of George IV, was rumored to worry about someone claiming the throne as a secret child of George IV.
https://en.wikipedia.org/wiki/William_Stourton,_18th_Baron_Stourton1
James Ord, father of US general Edward Otho Cresap Ord, was rumored to be the son of George IV and Maria Fitzherbet.
https://en.wikipedia.org/wiki/Edward_Ord2
But until and unless some proof of any such skullduggery and hanky panky turns up, we have to accept that the British throne has passed from rightful heir to rightful heir for three hundred twenty nine years since 1688. This compares to 340 years of legal succession in Denmark from 1523 to 1863. Thus the present monarch, Queen Elizabeth II, is the rightful heir of all previous monarchs back to the invading and usurping pair William III and Mary II. And only that far back, unless one totally accepts the ideology used to justify their invasion and usurpation.
If someone does not totally accept the ideology used to justify the invasion and usurpation of William III and Mary II, then one will tend to believe that Duke Franz of Bavaria is the rightful heir by genealogy of the Stuart, Tudor, Plantagenet, Norman, and Bruce Dynasties that ruled England from 1066 to 1688 and Scotland from 1306 to 1688.
https://en.wikipedia.org/wiki/Jacobite_succession3
Though alternate claims could be made to be the rightful heirs of some of those English or Scottish monarchs.
https://en.wikipedia.org/wiki/Alternative_successions_of_the_English_crown4
http://my.raex.com/~obsidian/Britpret.html5
And that the heirs of John I Balliol are by genealogy the rightful heirs of King Malcolm III of Scotland and his successors until 1290, and also the rightful heirs of the Anglo-Saxon kings of England of the House of Wessex.
https://en.wikipedia.org/wiki/Competitors_for_the_Crown_of_Scotland6
http://historum.com/european-history/125841-heirs-john-balliol-king-scotland.html7
(see posts numbers 5, 25, 27, & 28)
And that Karl von Habsburg is probably the rightful heir by genealogy of King Harold II Godwinsson.
http://my.raex.com/~obsidian/Britpret.html5
There is a complication here. Emperor Franz Joseph I (1830-1916) was probably the heir of Harold II Godwinsson. But Emperor Franz Joseph I (1830-1916) has three possible heirs.
By the rule of male preference primogeniture, the rule that passed the heirship of Harold II down to Franz Joseph, his heir is Guillaume, Prince of Windisch-Graetz (b. 1950).
http://europeandynasties.com/descendants_of_crown_prince_rudo.htm8
By the rule of male only primogeniture, or agnatic primogeniture, the heir of Franz Joseph is George, 3rd Duke of Hohenberg (b. 1929).
https://en.wikipedia.org/wiki/Hohenberg_family9
But George's grandparents, Archduke Franz Ferdinand and Sophie Duchess of Hohenberg, had a nondynastic, unequal, or morganic type of marriage, making their descendants unable to inherit the Austrian throne. That makes the right to the Austrian throne pass to Karl von Habsburg, the third potential heir of King Harold II Godwinsson.
Some historians have claimed that King Harold II Godwinsson was descended from King Aethelred, the older brother of King Alfred the Great, which would make the heirs of Harold II the rightful heirs of Anglo-Saxon England, if true.
And Mr. Evan Vaughan Anwyl of Tywyn (born 1943) may be the rightful heir of the kings of Gywnedd and Kings of the Britons.
https://en.wikipedia.org/wiki/House_of_Aberffraw#Anwyl_family_of_Tywyn10
Anyway, it doesn't matter to the present Queen Elizabeth II whether any royals reigning before 1688 were of legitimate or illegitimate birth. Her claim to the throne begins with the events of 1688.
As another example, in 977 King Lothair of France's wife Queen Emma was accused of adultery with Bishop Adelberen of Laon by Lothair's brother Charles. Nobody knows if the charges were true. Emma and Adelberen were not convicted and Charles was exiled from France. If the charges were false Lothair and Emma's son Louis was the rightful heir to the throne, if the charges were correct Charles would be the rightful heir to the throne.
In 978 Lothair invaded the Holy Roman Empire and Emperor Otto II retaliated by invading France and proclaiming Charles the new king. And no matter who was the rightful king or heir of France before, the instant that the emperor decreed that Charles was the king of France, all right to the throne was snatched away from Lothair and his descendants and/or successors - such as the Capetians and Bourbons - and granted to Charles and his heirs forever, or until such time as some future emperor should decree otherwise.
So it doesn't matter if Emmas's son, the future Louis V, was really Lothair's legitimate son or the bastard son of Bishop Adelberen of Laon. When the emperor decreed that Charles was the king of France, Louis and all his successors lost all right to become king.
Centuries later four future successors of Lothair and Louis married two sets of sisters, two daughters of the Count of Burgundy and two daughters of the Duke of Burgundy, just to confuse everyone.
The future Louis X, Philip V, and Charles IV, sons of Philip IV the (un) Fair, and their first cousin the future Philip VI, were married to those four young women when three of the wives were accused of adultery in the Tour de Nesle affair in 1314.
Posts # 1, 14, here: http://historum.com/medieval-byzantine-history/121828-navarre-succession-crises-1316-1322-1328-a-2.html discuss the possible lines of succession to the potential heirs.
And none of these possible cases of royal illegitimacy matter to the present day presidents of France whose right depends only on being elected according to the terms of the constitution of the Fifth Republic of France established in 1958.
And for much the same reasons the claim of Elizabeth II to be the rightful monarch of the United Kingdom does not depend on whether any medieval monarchs of England or Scotland were of legitimate birth. Her claim of right to be queen goes back only as far as the invasion and revolution in 1688. She claims to be the rightful heir according to the rules and laws of succession established by the invaders and revolutionaries of 1688, no more, no less.
If someone accepts that the "Glorious Revolution" of 1688 was totally right, then Elizabeth II is the 100 percent legal and rightful sovereign of the United Kingdom. If someone does not accept the "Glorious Revolution" of 1688 was totally right, then there are numerous other persons with claims to be the rightful monarchs, and the question of which medieval English and Scottish royalty was of legitimate birth is then important to selecting the true rightful heir.