This story comes from an Anglo-Saxon poem, which was probably written sometime between the years 700 and 800 A.D
The Anglo-Saxon people began to settle in Britain around 450 A.D. They came from Europe, mainly from the countries we now call Germany and Denmark.
Beowulf is written in an Anglo-Saxon Language (sometimes called Old English), but the the story does not take place in England at all. It is about some of the ancestors of the Anglo-Saxons, tribes who lived in countries that are now called Denmark and Sweden.