'William the Conqueror' is not 'William the Conqueror', he is 'Guillaume le Conquerant'. William himself spoke no English. Put simply the anglicisation of his name masks the 'frankicisation' of England. What happened after 1066 really changed England from 'Anglo-Saxon' with Danish and other Scandinavian influences into a country largely controlled by nobles, etc who spoke French and very little English. The re-anglicisation of the rulership England would take about another 400 years. This, however, did not stop English people from conducting their daily affairs in English including legal and other matters.
"Norman, member of those Vikings, or Norsemen, who settled in northern France (or the Frankish kingdom), together with their descendants. The Normans founded the duchy of Normandy and sent out expeditions of conquest and colonization to southern Italy and Sicily and to England, Wales, Scotland, and Ireland." (Britannica)
The Norman conquest and French immigration by Professor Elisabeth van Houts, Honorary Professor in European Medieval History, University of Cambridge
The story of French immigration into England as a result of the Norman conquest in 1066 is best told in two parts. First there was the military invasion and conquest, accompanied by violence and warfare, which resulted in the forceful occupation of England by a small aristocratic military elite. This initial phase was then followed by a period of approximately three generations, or 100 years, during which increasingly more people, of various social statuses, from western France settled in England, and ultimately became absorbed into the English population.
Norman-French migration to England was never a mass immigration of destitute people fleeing their lands in search of work and a new life. This was a migration caused by a military event and political takeover of England by an ambitious French warlord with a band of military followers. Although William the Conqueror (1027-1087) claimed the English throne on the grounds of consanguinity there is no doubt that he was attracted by the wealth of England. The kingdom was rich in mineral resources and the most centrally administered country in western Europe. Its wealth in taxes and income from land created direct income for the king, who in turn used it to reward his followers.
Thus, the Norman Conquest of England in 1066 was a military takeover of England by William, Duke of Normandy. He had arrived uninvited with a large invasion fleet and army consisting of Norman, Breton, Flemish and other French soldiers. Numbers are hard to come by but a conservative estimate is that the Conqueror’s army comprised approximately 5,000 to 8,000 men, of whom twelve became the richest landholders in England. This is a relatively small number given that the estimates of the English population suggest a number between 1.5 and 2 million indigenous people.
source: https://www.ourmigrationstory.org.uk/oms/the-norman-conquest-of-england-women-invasion-and-migration
Conquered and conquerors
Queen Emma of Normandy brought a Norman-French influence to England with her marriage to Ethelred in 1002. Her son, Edward the Confessor, brought a number of Norman nobles and advisors over to England when he became king in 1042 after Cnut’s son Harthacnut died.
Then, in 1066, the ruler of Normandy, Duke William, decided to make his claim to the kingdom of England through conquest. He had some blood relation to the dead king, Edward the Confessor, through Emma, the late Queen Mother, and he claimed that Edward had promised him the succession before he died.
William’s forces invaded England around the same time as a Norwegian force under Harald Hardrada also invaded. England’s ruling council, the Witan, had chosen an English noble, Harold Godwinson, as king, so Harold had to fight both the Danes and then the Normans to keep the throne.
He defeated the Danes at Stamford Bridge, but then lost to William of Normandy at the Battle of Hastings in October 1066. This famous military event led to the migration of many Norman-French nobles who joined those who had come earlier with Emma and Edward. The land of England was divided up and given to these nobles by the new King William I, also known as William the Conqueror.
source: https://www.bbc.co.uk/bitesize/guides/z3wfmnb/revision/2
The Norman conquest: women, marriage, invasion by Professor Elisabeth van Houts, Honorary Professor in European Medieval History, University of Cambridge
In the wake of victory, those who were married brought their wives and children to England from France. But many soldiers were unmarried and were not easy to control as a social group. Indigenous English women became a vulnerable group as a consequence.
Intermarriage: a tool of peace negotiation
Women who had inherited family land because their husbands or fathers had been killed at Hastings in 1066, were only allowed to keep their family land if they married a Norman or Frenchman. In a sense such marriages were alliances, established to further the post-conquest social order and peace in England.
Although most intermarriage took place at the highest aristocratic level, to broker peace deals, there were several instances of intermarriage at middling noble level. At lower social, but still noble, level we find mostly foreign men marrying English women.
Assimilation and acculturation
By the mid-1180s, well over one hundred years after the Norman Conquest, the author of the Dialogue of the Exchequer, a manual for the workings of the Exchequer (the ‘finance department’ of English royal government) refers to widespread intermarriage amongst the people of free birth as a result of the Norman Conquest:
‘...but now with the English and the Normans living side by side and intermarrying, the two nations are so mixed that today one can scarcely distinguish who is English and who is Norman – among free persons, that is’.This small group of migrants was gradually absorbed into the settled English population.
England Pre-Norman Conquest Surnames (by The National Institute for Genealogical Studies, found at familysearch.org)
"It is a common misconception that people with French-sounding names must have either come over in 1066, or were Huguenots. There were actually several different influxes of French surnames into Britain. The largest number of surnames introduced by the Normans were from their castles or villages in Normandy. Arundel, Bruce, Clifford, Devereux, Glanville, Mortimer, Mowbray, Percy and Warren come to mind as well as the forms that retained the preposition such as de Courcy and D’Abernon. Some developed into more than one form, including Dolley and Olley from Ouilly, and Turberville and Durbeyfield from Thouberville (Reaney 1967). The Bretons who fought at Hastings chose to settle largely in East Anglia which is the seat of the surnames Brett and Britton. They left their mark with other surnames such as Allan, Harvey, Jekyll, Jewell, Mingay, Sampson and Wymark. The union of England and France continued under the Plantagenet kings into the 15th century. There was trade and the crusades which assisted emigration of Frenchmen into England, bringing with them locative surnames such as Burgoyne (Burgundy), Gascoigne (Gascony), Mayne (Maine), Peyto (Poitou), Picard and Poor (Picardy)."
"In 1066 William Duke of Normandy defeated the last Anglo-Saxon King, Harold, at the Battle of Hastings. Norman rule transformed the English language and culture, but there is no genetic evidence to suggest that more than a small number of elite families settled in Britain."
(source: Oxford University Museum of Natural History / University of Oxford (http://www.oum.ox.ac.uk/settlers/)
"Given the cultural significance of the Roman, Viking and Norman invasions, it’s surprising they didn’t leave greater genetic legacy. For the Romans and Normans, that may be because they were ruling elites who didn’t intermarry with the natives.
The overall message is that despite their large cultural impact, Britain’s main invaders left no genetic stamp of note. “When you study the past through history, linguistics or archaeology, you learn about successful people,” says Donnelly. “History is written by the winners, so much of current historical information is from a relatively small subset of people. Genetics, by contrast, is the history of the masses.”"
(source: https://www.newscientist.com/article/mg22530134-300-ancient-invaders-transformed-britain-but-not-its-dna/#ixzz6vnH5oNst)
"Despite the huge significance of the Norman conquest of 1066, the numbers of Normans that followed William the Conqueror to England are accepted by most historians as being small. Indeed, one historian states that ‘only ten thousand or so Frenchmen followed in William’s footsteps-less than one percent of the population.’[18] Other Historians point to higher inflows, with some estimating that Norman settlers eventually made up as much as 5% of the population.[19] Regardless of the size of their migration, the Normans made a tremendous impact on England; quickly becoming the country’s governing elite and biggest landowners".
(source: A summary history of immigration to Britain (https://www.migrationwatchuk.org/briefing-paper/48/a-summary-history-of-immigration-to-britain))
The Norman Conquest
Immigration and intermarriage
An estimated 8000 Normans and other continentals settled in England as a result of the conquest, although exact figures cannot be established. Some of these new residents intermarried with the native English, but the extent of this practice in the years immediately after Hastings is unclear. Several marriages are attested between Norman men and English women during the years before 1100, but such marriages were uncommon. Most Normans continued to contract marriages with other Normans or other continental families rather than with the English. Within a century of the invasion, intermarriage between the native English and the Norman immigrants had become common. By the early 1160s, Ailred of Rievaulx was writing that intermarriage was common in all levels of society.
Article by Mark Cartwright, published on 23 January 2019
The lack of an influx of tens of thousands of Normans was no consolation for the Anglo-Saxon aristocracy, of course, as 20 years after Hastings there were only two powerful Anglo-Saxon landowners in England. Some 200 Norman nobles and 100 bishops and monasteries were given estates which had been distributed amongst 4,000 Anglo-Saxon landowners prior to 1066 CE. To ensure the Norman nobles did not abuse their power (and so threaten William himself), many of the old Anglo-Saxon tools of governance were kept in place, notably the sheriffs who governed in the king's name the districts or shires into which England had traditionally been divided. The sheriffs were also replaced with Normans but they did provide a balance to Norman landowners in their jurisdiction.
(source: https://www.worldhistory.org/article/1323/the-impact-of-the-norman-conquest-of-england/)
French migration to the United Kingdom (Wikipedia)
French migration to the United Kingdom is a phenomenon that has occurred at various points in history. The Norman Conquest of England by William the Conqueror in 1066 resulted in the arrival of French aristocracy ... Much of the UK's medieval aristocracy was descended from Franco-Norman migrants to England from the time of the Norman Conquest. Prominent families of the period, include the Grosvenor family originally, "Gros Veneur" (in Norman) "great hunter": their influence can be found throughout central London with many roads, squares and buildings bearing their family names, such as Grosvenor Square and Grosvenor House. Ancestors of the Molyneux family, the Earls of Sefton who arrived in England around the time of the Norman Conquest, bore the name "de Molines": they came from Molineaux-sur-Seine, near Rouen, in Normandy where they resided in the Château de Robert-le-Diable also known as Château de Moulineaux. Other well known names are the Beauchamps (Beecham), Courtois and Le Mesurier.
"Some, but not all of them, were titled nobility in Normandy and retained their holdings there as well; others were poorer men, who rose to power through their military or political service, and women, who gained power through their alliances and marriages into powerful families. Some anglicized their names over time, while others retained the French spellings and geographically-based surnames referring to their ancestral villages in Normandy."
This site does not discuss the 'de Guînes' family.