The British Isles haven't always been a separate part of Europe. Long time ago Britain was a part of the European continent. Then about ten thousand years ago during the end of the last Ice Age, when the climate grew warmer, new rivers and sees were formed Europe slowly moved into its present shape.
The ancient people of Britain were simple hunters and ate flesh of animals fruits, nuts, honey. They fished, and gathered oysters. They didn't have a permanent place of living and traveled from place to place, sheltering in caves.
Then the British men have become the farmers. The Stone Age farmer had the patches in the forests that covered most of Britain. He kept half-wild cattle and pigs in a forest and in Northern Scotland, free from forests, he kept sheep.
By the end of the Stone Age (2000 BC) metal was already being used. It was time of the Invasion of Beaker people. They came from the Northern Europe. They used bronze and it was them, who started the building of Stone monuments at Stonehenge. Mining and trade were growing during the Bronze Age (ab. 2000-500 BC).
About 500 BC British people were learning how to smelt iron. Iron tools had an advantage over bronze ones: they were much cheaper.
The beginning of the Iron Age (ab. 400 BC) British Isles were invaded by Celts armed with weapons of iron. They conquered Kent and much of Southern England. They imposed their language on the natives, its Gaelic form was used in Ireland and Scotland, the Brythonic form - in England and Wales. It was the Brythonic tribe that gave its name to the whole country.
The first chronicle of Britain was written by an educated merchant from Morsel. He also wrote the first description of the people, called Celts. He said they were a gentlefolk, skilled craftsman, who welcomed visitors. The most educated visitor of Britain described the British as a fierce race. His name was Julius Caesar.
Present English dates back to 5th-6th centuries, when Germanic tribes of Jutes, Saxons & Angles overran all England except Cornwall & Cumberland. Some religious terms were borrowed from Latin in connection with converting England to Christianity by St. Augustin. Some parts of England were invaded by Danes & Norwegians, that's why the languages of the Anglo-Saxons & Danes formed the basis of English.
Normans contributed greatly to the developing of English language during their invasion.
Next point of this was the 15th-16th centuries when written language was stabilized with help of spreading of printing. In 19th century the growth of British colonial power led to the spread of English as world language.
But still it was only the 1930 when the British Foreign Office stopped using French for all its official memoranda.
So It was the long way of coming-to-be the language of international communication from old Anglo-Saxon dialects to the "world language" in 20th century.