Magic is part of our world in many areas. However, there are some areas of Earth that seem to attract magic and fantasy more than others. One of these areas is known in modern days as Great Britain. The United States has similar qualities but to a lesser degree. It would increase in importance in the future.
After the events of The Lord of the Rings, the entire planet underwent several changes. Continents would move, some areas would flood, and in some areas, new land would arise. That is why it looks very different in different eras of time. Even the seasons were changed for a time, becoming longer. However, the basic shape and positioning of Great Britain was preserved.