The British Isles has had a number of different languages spoken in it over the last two thousand years. In fact, it probably had more different languages than the rest of western Europe. The only ones that come close are the Mediterranean ones that experienced a mix of Latin, Greek, and Arabic before settling on their modern language.
Originally the islands were Celtic speaking. Descendants of these languages persist in Ireland and Wales, and to a very small extent in Scotland and Cornwall. Starting in the first century BC, however, when Julius Caesar conquered Great Britain, the predominant language became Latin. Britain under the Romans was Latin-speaking and Christian, reading the Bible in Latin rather than the original Greek (much less the Hebrew in which the Old Testament was written before it became, in Greek, part of the Christian Bible--see more here on the Bible in late antiquity).
And then the Angles and Saxons showed up (on whom see more here). They spoke a version of German. In what is now England, named for the Angles (but not in the territories on England's margins or in Ireland, the same areas where Celtic languages still linger), both Latin and Christianity essentially disappeared, along with Roman culture.
But these Germanic speaking people were Christianized in the seventh century and by the eighth century were producing excellent scholars, very learned in Latin. Because for them Latin was a learned language, not an everyday spoken language, they were very careful about things like declensions and case endings and verb forms. Ironically, on the Continent, where Latin was still a spoken language, a lot of people thought they were speaking good Latin when, from a modern perspective, it was rapidly becoming Old French or Old Italian or Old Spanish. When Charlemagne's royal court took on some Anglo-Saxon scholars, they were quick to point out the difference between real Latin and what people were speaking.
Anglo-Saxon continued as its own valid language, getting a good written collection of books, including translations of the Bible into Anglo-Saxon, writing down of ancient laws, and the like. But everything changed abruptly with the Norman Conquest of 1066.
The Normans arrived speaking Old French, so Anglo-Saxon immediately became not a learned language but rather the language of conquered peasants. For the next two centuries Norman French and Anglo-Saxon German existed side-by-side. The same thing might be called two different things depending on who was talking about it. A cow (a Germanic word, related to "cattle"), the creature being raised by a peasant, became beef, a French word, once it reached the lord's table. (In modern French, boeuf still means both the animal and the meat.)
In the fourteenth century the two languages ended up merging, creating Middle English, the ancestor of modern English. There were a number of different Middle English dialects, but Chaucer, for example, can still be read by modern readers if there are notes on some of the words. Modern English, which has roughly as many words as modern French and German combined, came into its own in the so-called "Elizabethan age" on either side of the year 1600. This is the age of Shakespeare, who can now be read more easily than Chaucer, and of the King James Bible, sponsored by Elizabeth's successor, King James.
The King James Bible is still the most frequently used Bible in English. It was translated directly from the Hebrew (Old Testament) and the Greek (New Testament), without reference to the Latin (Vulgate) version that had been the standard in the Middle Ages. (Contrary to popular belief, the Bible was not written in English.) Once this language became the standard for religious service, the language stopped changing nearly as fast as it had earlier.