Why did Latin disappear so completely in Britain after the Romans left?

In my understanding, Latin was never anyone's first language outside of the Italian peninsula.

It was, of course, the lingua franca for intra imperial trade and officialdom, but anything local was conducted in the local languages. I think the closest modern day equivalent in modern times would be India. Each region has its own local language or dialect. However, for any wider trade across India, you would need to speak Hindi or English.

Because Britain is an island, and because of the sheer distance from the remains of the empire after it withdrew from Britain, Latin was no longer useful to know, and was more or less forgotten. Naturally, there are exceptions, mostly for priests and lawyers.

England went through a few invasions (Saxon, Anglo-Saxon, Danish, and Viking) that saw some exceedingly brutal practices. Jury is still out and there are a few competing theories, but it's quite possible Roman-English descendants experienced what we would now refer to as genocide.

https://harvardmagazine.com/2009/07/who-killed-the-men-england

Using computer analysis, the researchers explored how such a pattern could have arisen and concluded that a massive replacement of the native fourth-century male Britons had taken place. Between 50 percent and 100 percent of indigenous English men today, the researchers estimate, are descended from Anglo-Saxons who arrived on England’s eastern coast 16 centuries ago. So what happened? Mass killing, or “population replacement,” is one possible explanation. Mass migration of Anglo-Saxons, so that they swamped the native gene pool, is another.

Yet no archaeological or historical evidence from the fifth and sixth centuries hints at the immense scale of violence or migration that would be necessary to explain this genetic legacy. The science hinted at an untold story.

There was a massive change genetically in England that suggests a huge number of the native population was killed off. This is relatively unique, especially within Europe. Quite literally, the majority of 'Roman English' speakers were killed off and their language wasn't able to influence Enlgish as heavily as it did the Romance languages.

Generally speaking vulgar latin remained within the limes, and disappeared outside of it. The main exceptions are Batavia and Britain. Batavia was as far as I know evacuated by the Romans. Apparently Britain was not so intensively Romanised as other regions of the empire. The mass of the people continued to use their native languages.

IMHO the elite in Roman Britain spoke Ancient British and/or classical Latin, many or most speaking both. The lower classes spoke vulgar British - which was beginning to evolve into early proto Welsh and related languages - and/or vulgar Latin, many speaking both. And in households where the lower classes served the upper classes the people of each class would learn the dialects of the other class at least a little.

This continued for decades in Post Roman Britain. Then the Saxons invaded and settled. In areas taken over by the Saxons, Britons had to learn enough of Old Germanic to understand the orders of their new Saxon masters, and British and Latin died out swiftly.

In lands that were not conquered by the Saxons, British evolved into Cumbrian, Welsh, Cornish, and Breton, and Latin continued to be used as the language of the church and to a degree in secular society.

Gildas wrote The Ruin of Britain sometime between about 480 and 580, not to be overly precise, a few generations after the beginning of the Saxon invasions, and yet his Latin was very good, and he expected his target audience - however large or small it was - to understand.

There are many Latin inscriptions in the western parts of Post Roman Britain. The making of Latin inscriptions survived the longest in the Kingdom of Gwynedd in the remotest part of Wales, perhaps because the Kings of Gwynedd claimed to be the Kings of the Britons, rightful rulers of all Britain, and maybe also claimed to be Roman Emperors of a sort, successors to the rights of Constantine III.

I agree with at least most of J. Siebeneichler's & Sempaiscuba's answers and add the following.

It is true that as much of Western Europe Latin continued in use in the British Isles after the end of the Western Roman Empire within the Church, and hence for ‘book learning’ generally, which was for a time mostly a church thing.

However, the Geoffrey Thomas's original question is right that as a language of everyday life and as people's first language Latin quickly and disappeared in Britain, unlike in almost all other ex-provinces of the Western Roman Empire, and that this is important and requires explanation.

The only Pre-Roman languages that survive in the former Western Empire are Welsh (and its ancient offshoot Breton) and Basque. (The East, over much of which Greek was the main language of government and trade, has a different linguistic history)

In the mainly lowland parts of south and east Britain that were conquered by the Anglo-Saxons and became England, Germanic dialects prevailed, and developed into the English language.

In the mainly upland parts of the west where the native peoples initially avoided conquest, the pre-Roman native Celtic language Welsh (initially just called 'British') predominated.

The question may therefore be divided into:

1. Why did the Anglo-Saxon incomers not adopt a local dialect of late Latin as e.g. the Franks eventually did in Gaul, the Lombards in Italy or the descendants of the Visigoths and Sueves in Spain and Portugal? (Such dialects gradually becoming the newly differentiated but almost all Latin-based languages such as French, Languedoc, Italian, Castilian and Portuguese.)

2. Why did the Celtic language speaking native population of former Roman Britain either remain Celtic (Welsh) speaking or else adopt English? There was no equivalent to e.g. formerly Celtic-speaking Gauls adopting the late Latin dialect that became French.

We do not know the details of how this happened as the key period c 400 - 600 AD is one of the most obscure in British history, from which almost no written records survive, apart from one short book/ sermon 'On the Ruin of Britain' by a monk called Gildas. Attempts to reconstruct the history of the formative period of England therefore rely heavily on works written centuries later, or the rare references to Britain by Continental European writers of the time.

It is possibly significant that:

-Britain was only permanently occupied by the Romans from 43 AD, a century or more later than e.g. Gaul, Spain or northern Italy. Roman rule in Britain seems to have ended abruptly around 410 AD, earlier than most of the rest of the Western Empire. Roman civilization and Latin language thus had less time to become generally adopted.

While it is hard to prove or disprove, I have heard an archaeologist seriously suggest (in a talk to a Historical Association) that the effects of being 'last in, first out' of the Roman Empire are felt even today in Britain’s ambivalence about being 'European'.

However, this argument must be qualified. Britain was under Roman rule for more than 400 years, quite a long time; about twice as long as the United States has so far existed and more than twice as long as, say, British rule lasted in India.

-Possibly a more important reason for the very early disappearance of Latin as an everyday spoken language in Britain, unlike in e.g. France or Spain, was that the Anglo-Saxon influx into Britain differed from the Germanic influxes into France and Spain in that the Anglo-Saxons were still pagan and largely illiterate (despite some short Runic inscriptions). They had had less contact with the Roman Empire than say the Franks and Visigoths by the time those peoples conquered France and Spain (both Franks and Goths had by then already adopted a form of Christianity). Accordingly it was easier and more inviting for the Franks and Visigoths, already somewhat more Roman influenced and having more understanding of how Roman government and society had worked, partly to preserve and slowly to merge into the Latin-speaking societies they had conquered. The pagan Anglo-Saxons were more likely just to destroy and replace.

-Also, the south and east of Britain, closest to the rest of the Roman Empire and, to judge by the number of Roman period towns and villas were by far the most Romanised parts of Britain, and hence the parts most likely to have adopted Latin speech.

Further north and west, places like the hills and mountains of what are now Wales, Cumbria and Cornwall show less sign of Romanisation, apart from having more Roman army forts, presumably to keep the still half-barbarian inhabitants in subjection, as well as to keep out even wilder barbarians from what are now Scotland and Ireland. Consequently, although individual Latin words were absorbed into Welsh, native British Celtic speech probably persisted more strongly in these areas.

It was the misfortune of any Latin-speaking Romano-Britons there may have been in the South and East of the island that their regions, having the richest farmland and being closet to continental Europe, were the ones most quickly and thoroughly conquered by the incoming Anglo-Saxons, and hence were the most complete language replacement occurred.

-I have also read what I think was only a half-joking suggestion by a professional historian, I cannot remember where unfortunately, that following the Anglo-Saxon Conquest most of Britain became purely English speaking because 'the English have never been good at learning foreign languages'!

Well, Latin did not entirely disappear from Britain. The Latin language would continue onward through the Middle Ages as the Central Language of the Catholic Church-(and Latin may have continued into the time of King Henry VIII. However, since the establishment of The Church of England, English would emerge as the dominant Ecclesiastical language).

Although Ecclesiastical Latin survived well into the Middle Ages throughout many parts of Britain, with the arrival of the Anglo-Saxons beginning around the 400's AD/CE, as well as the subsequent abandonment of the Romans at around the same time, a new peoples, culture and yes, a new language, would redefine Britain.....the English language.

The very name, "England", is a Medieval Saxon /Germanic word which roughly translates as, "Land of the Angles". Although the ancient name of "Britannia" has survived into the current age, "England", became the new home of the Germanic Angles and Saxons, who in turn, replaced the Roman imperial presence-(including, the widely spoken centuries old Latin language) and relocated many of Britain's indigenous Celtic inhabitants elsewhere-(most likely towards Wales). By the 500's AD/CE, old Celtic and Roman Britannia, would become history and Germanic England, more specifically, the English language, was born.

(Note: Keep in mind that the earliest known form of English, dating to the "Beowulf" era, sounds more like a Scandinavian or Germanic language than even the English of Chaucer's time. Early English would sound nearly unrecognizable to a contemporary English ear, unless they are students of English linguistics, English linguistic history or Early English Literature......the era of "Beowulf" and Bede).

It had to do with the way that "Britain" was settled by the Anglo-Saxons. The general idea was that they "swamped" the locals, but there are at least two variations on the theme.

Variation one is that the locals had been weakened by natural disasters and internecine fighting, perhaps occasioned by the weakening Roman influence in the fifth century CE. When the Angles and Saxons arrived from across the North Sea, they pretty much "wiped" out most of the remaining locals (except in remote areas like modern Scotland and Wales) without ever absorbing a meaningful part of their "Latin" culture.

Variation two is the that the invaders thoroughly subjugated the locals without "wiping them out." But they imposed a cultural imperialism, kind of like what the (German) Teutonic knights did to the Baltic Prussians.Anglo-Saxons so dominated the locals that the survivors were "absorbed" into dominant group without having a meaningful influence on them in their turn.

What did not happen was that invaders co-existed with the locals on more or less equal terms and "shared" their cultures as happened with e.g. Germans and Latinized Gauls in modern France.

(This answer is intended as a complement to Sempaiscuba's.)

The British experience is not as unique as you may think, as there were many other places where Vulgar Latin died out after the collapse of the Roman Empire:

• in Africa, a so-called "African Romance" (which was probably multiple languages), was eventually replaced with Arabic and Berber languages (with which it coexisted);
• Around Lake Balaton in modern Hungary, a "Pannonian Romance" was spoken;
• In western Germany, Moselle Romance was spoken;
• The native Romance language(s) of central and southern Iberia, Mozarabic, was partially replaced by Andalusian Arabic. During the Reconquista, it was replaced by other Romance languages.
• Much more recently, Dalmatian, the native Romance language of the eastern Adriatic coast, has been replaced by Slavic languages.

There are also other places where the native Romance language has been mostly, but not completely replaced:

• In Istria, Istriot is nearly dead, having been replaced by Istro-Venetian (which is nearly dead as well) and Croatian.
• In the Alps, a group of Rhaeto-Romance languages remains in use.
• In the southern Balkans, Aromanian is spoken here and there. The Durmitor mountain in Montenegro has a Romance name, even though Romance is no longer spoken there.