As a living language Latin inevitably developed and changed while it was used all over Europe during the thousand years that we term the Middle Ages. Inevitably, too, it was used, and developed, in particular ways during the medieval period in Britain, as the result of changes in society to which it had to adapt.
At the beginning of this period, the Latin used in these islands does seem to have been predominantly the language of the church, in the form of a Latin recognizable from classical texts but liberally larded with the language of the Latin Bible text, as seen in Gildas' polemical account of recent events in Britain, written in about 550. But even Gildas uses words that are not found elsewhere, such as semidormitare (‘to be half asleep’), a compound of two words found in Classical Latin, semi and dormitare. Already at this stage we see that those who use Latin, whether for writing or perhaps also for speaking even at a time when the Romans have officially abandoned the country, do so surrounded by speakers of other languages. Writing of the coming of the Saxons to England some hundred years earlier, Gildas introduces the word cyula giving a Latin form to the Saxon ciol, which he explains as meaning ‘long boat’. And yet Gildas himself is likely to have grown up speaking some form of Celtic language, and indeed Latin in medieval Britain was to see the influence not only of Saxon (as it developed over the centuries into what is termed Old English) and Celtic languages, but also of Norse (with the coming of raiders and settlers from Scandinavia in the centuries after the destruction of Lindisfarne in 793), French (after the Norman conquest in 1066), and Arabic (with the discovery of philosophical and scientific texts from the eleventh century). Greek too continued to be an important influence on the language, as it had been since classical times.
While British Medieval Latin was certainly subject to numerous influences, which give it a distinctive character and represent its vitality, we might note that it was also highly regarded in its own day for its quality and standard. In particular, British Medieval Latin played a key role in mainland Europe in what might be thought of as the formal birth of some of the Romance languages. By the time of Charlemagne in the eighth century there had arisen considerable mutual influence (and interference) between the Latin language and the diverging everyday spoken Romance languages. Charlemagne's institution (or rather his development of his father's programme) of educational and cultural reforms brought about a clearer recognition of the separation between Latin and everyday language as a result of purifying the Latin language to bring it back towards its former classical and patristic heyday. The key source for this new purer Latin came from Britain, where the everyday languages were not themselves descended from Latin and their effect on the Latin quite different in quantity and nature. While Italian scholars were also influential, perhaps the most significant figure in the Carolingian renaissance, from a linguistic perspective, was the Northumbrian monk, Alcuin, who went from York to Charlemagne's court as a key advisor and for many years headed the Palace School in Aachen.
Latin in Britain was primarily a written language, used for communication by letter, for theological and educational texts, for administration and records, but it was also spoken and sung mostly in churches and monasteries from Biblical texts and in the words of the liturgy and also in monastic schools, as we see in the Colloquies of Ælfric and his pupil Ælfric Bata (writing around the year 1000), in which the children are taught, often through the use of humour, to converse colloquially in Latin, helped by an almost word-for-word Old English translation alongside the Latin text: if people were to speak Latin as an everyday language, they had to be given the words for everyday items and activities, for food, domestic and agricultural tools, animals etc. in what amounted to a beginners’ course in Latin as a foreign language.
As society in Britain developed, bringing technological, social, and administrative changes, new words had to be invented or borrowed so that people could, for instance, refer to each part of the machinery of a mill, to parts of ploughs and carts and ships, or to the enormous variety of feudal dues and taxes when they were writing financial accounts, charters or legal records. As a result we find new words such as essewera meaning a drain, ditch or weir, ultimately deriving from Classical Latin ex and aqua and leading by way of Anglo-Norman and Old French forms of the word to the modern English word sewer.