Were there any monarchs of England or the United Kingdom who were not technically first in the line of succession?

This has happened more than once. One of the earliest incidences was King Stephen. When his predecessor Henry I's heir died, he settled on his daughter Matilda as heir. At the time of his death, Matilda could be seen by that right as being "first in line of succession". But with the inherent misogyny of the time, England was not ready for a queen regnant, and so the country fell into civil war, and it was ultimately King Stephen who gained power.

Lady Jane Grey (possibly reluctantly) attempted to upset the line of succession stipulated by Henry VIII

• being the Nine Days' Queen from 10 July until 19 July 1553

After the death of Edward VI (age 15), his sister Mary Tutor, based on the Third Succession Act of July 1543, was the intended successor.

For political/religious reasons having a non Protestant as Monarch was considered undesirable.

Edward VI meant to bypass this Act in his "Devise for the Succession", issued as letters patent on 21 June 1553, by naming Lady Jane Grey as his successor in place of Mary. Prevailing over Lady Jane Grey, Mary ascended the throne under the terms of the Third Succession Act.

In September [1553], Parliament declared Mary the rightful successor and denounced and revoked Jane's proclamation as that of a usurper.

Lady Jane Grey was tried and found guilty of high treason.

On the morning of 12 February 1554 her beheading took place at the Tower Green.

One could also say that Mary I was not the intended successor of Edward VI.

Then, as now, peaple were flexible

• depending on which direction the wind blows

Are you asking if there were any usurpers in English history, when someone overthrows and replaces the ruling king? Or a person who is not the rightful heir of the king according to normal inheritance customs becoming king when the old king dies.

Both have happened in English history.

An example of the second is when King Richard I died in 1199. His nephew Arthur, duke of Brittany, son of Richard's deceased brother Geoffrey, was the rightful heir according to the usual inheritance custom, male preference primogeniture, while Richard's youngest brother John was the closest relative. Richard switched between choosing Arthur or John as his heir a few times. When Richard died John became King of England, Duke of Normandy and Aquitaine, etc., and Arthur disappeared a few year later while John's prisoner.

The kingdom of England is sometimes said to have officially begun in 927, and it ended after 780 years in 1707 when England and Scotland merged to form the Kingdom of Great Britain.

And I guess that there were about a dozen or so cases when when someone not the customary genealogical heir became king when the previous king died, or even usurped the throne from a living king.

The first time was in 1013 when King Sweyn Forkbeard of Denmark invaded and became King of England. King Aethelred the Unready fled to Europe. Sweyn died in 1014 and Aethelred regained the throne. But Sweyn's son Canute the Great invaded England and fought against Aethelred. When Aethelred died in 1016 his son Edmund Ironside became king and fought Canute.

Edmund and Canute decided to divide the kingdom between them. Edmund then died of natural causes or maybe murder, and Canute became the sole king of England in 1016.

When Canute died in 1035 his son Harthacnut was his heir but another son, Harold I Harefoot, became king of England. Harthacnut became king of England after Harold died in 1040 but died in 1042, possibly poisoned by Edward the Confessor, a son of Aethelred the Unready.

Edward the Confessor became king of England in 1042, and by then there had already been a few more usurpations of the English throne since 1013, though it is hard to decide how many successions count as usurpations.

Edward the Confessor had no children, but in 1056 he learned that Edward the Exile, a son of Edmund Ironside, was living in Hungary and invited him to England. Edward the Exile, died in 1057 right after arriving, possibly murdered, and had a young son Edgar Aetheling (c.1051--c.1126), who was the only surviving member of the House of Wessex and the genealogical heir to the throne.

The powerful noble Harold Godwinson, Duke William of Normandy, King Sweyn II of Denmark, and King Harold Hardrada of Norway all wanted to become king of England, although King Sweyn II of Denmark was the only one of those greedy men descended from a previous king of England, and that was only from King Sweyn Forkbeard, a foreign invader and usurper.

King Edward the Confessor died in 1066, and Harold Godwinson got himself selected king by the nobles of England. King Sweyn II of Denmark was unable to invade in 1066, but he did so in later years. Harold Hardrada invaded and was killed at Stamford Bridge, and William the Invader invaded and defeated and Killed Harold II Godwinson at Hastings. Edgar Aetheling was selected king at London, but everyone submitted to William the Usurper as he marched on London.

And there have been a number of other usurpations of the throne from 1066 until the final one, the "Glorious Revolution" in 1688. Thus I can imagine a prankster putting up a sign at Buckingham Palace saying something like "Home of the British Monarchy. No usurpations for 321 years since 1688".

This site lists potential pretenders or claimants of the English throne and other British thrones by tracing the genealogical heirs of various kings, dynasties, and claimants down to the present.

Wikipedia has an article, Alternate succesions of the English and British Crown.

According to Wikipedia's list of English Usurpers, there were only seven usurpations of the English crown. But they start with William I in 1066 and leave out John in 1199 and the deposition of Edward II by Queen Isabella and Roger Mortimer in 1327. Isabella and Mortimer made Edward III king, which makes him technically a usurper even though he wasn't involved in the plot.

I note that King George IV (reigned 1820-1830) of the United Kingdom was sort of a bigamist when Prince of Wales. This makes it hard to determine which - if either - of his two marriages was fully legal.

George IV had only one child by his public marriage, Princess Charlotte, who gave birth to a stillborn child and died soon after, during the lifetime of her grandfather King George III.

George IV's estranged wife Queen Caroline was very fond of a boy which some people suspected was an illegitimate child of hers. If it could be proved that the boy was her son, everyone would believe he was illegitimate and not a heir to the throne. But their beliefs might not have mattered since in British law a child born while the mother is married is presumed to be the son of the husband without strong evidence to the contrary. So if that boy was the son of Caroline, he might have been the legal heir to the British throne.

There are no known children of the "secret" marriage of George IV with Maria Fitzherbert, and it was legally dubious that any children of that marriage would have any claim on the throne.

But there were rumors about possible children of George IV and Maria Fitzherbert, and it is said Queen Victoria (reigned 1837-1901) worried that a child of George IV would claim the throne. And there may still be families descended from alleged children of George IV and Maria Fitzherbert.

So because of the behavior of George IV, Queen Caroline, and Maria Fitzherbert, I am only about 99 percent certain that Elizabeth II is the totally rightful heir of King William III and Queen Mary II according to the Act of Succession and other laws.

This is what the War of the Roses was about.

The Houses of Lancaster and York fought each other until the Lancaster branch went extinct. For a moment it seemed like things may have ended there -- which is to say, England moving on, with an usurper on the throne. But the House of Tudor inherited the Lancastrian claim, and its supporters ultimately prevailed.

Many, including William II, Henry I, Stephen, Henry II, John, Henry IV, Edward IV, Richard III, Henry VII, William III, Mary II, Anne, George I (i.e., were not next heir of previous monarch by primogeniture). Richard III is squishy. It depends whether Edward V and his brother were already dead.

Note that I was explicit about how I interpreted "line of succession". Several of these monarchs, notably Henry II, John, and Anne (and, arguably, George I) had been recognized as the successor by the preceding monarch and so one wouldn't call them usurpers even though they weren't next of blood. Others such as Henry IV and Edward IV had only been recognized by their predecessor under duress. Stephen, Henry VII, William III, and Mary II had clearly in no way been recognized by their predecessor.