But the events of Beowulf happened in Scandinavia. The poem itself was written in England, though. Probably.
Not that I think it matters much: I suppose only those cultures that have existed entirely in isolation really have truly unique myths, and that isn't very many cultures. And maybe not all of those unique cultures, either, given how many themes are found in the mythology of very different peoples.
Right, which presumably means that the tale originated before the Saxons invaded Great Britain. But nonetheless the version we know is told in Old English and not any of its continental predecessors, and its earliest manuscripts are all found in Britain. It isn't disqualified from being uniquely English by the fact that it derives from earlier times in Scandinavia any more than Mormonism is disqualified from being uniquely American by the fact that it derives from earlier times in Jerusalem.
Not that I think it matters much: I suppose only those cultures that have existed entirely in isolation really have truly unique myths, and that isn't very many cultures. And maybe not all of those unique cultures, either, given how many themes are found in the mythology of very different peoples.