The use of the English language: Given that the story takes place in England during the Hundred Years' War, it would make sense for the characters to speak English. This could be a way to showcase the language and its evolution over time.
The inclusion of historical figures: The Hundred Years' War was a significant event in English history, so incorporating real-life historical figures into the story could help to ground it in its British setting. Characters like King Henry V, Joan of Arc, and the Duke of Burgundy could all make appearances.
The depiction of English culture: Whether it's through descriptions of food, clothing, or social customs, including details about English culture can help to create a sense of place and time. For example, characters could eat traditional English dishes like roast beef or fish and chips, or they could engage in activities like archery or jousting.
The use of British landmarks: Including famous British landmarks like Stonehenge, the Tower of London, or Buckingham Palace could help to ground the story in its setting and add a sense of familiarity for readers.
The portrayal of British values: The story could explore themes like honor, duty, and loyalty, which are often associated with British culture. Characters could wrestle with questions of morality and ethics as they navigate the challenges of war and fighting supernatural enemies.