One reason could be to find peace. After all the chaos and war in England, they might just want to get away from it all and start fresh somewhere else.
They could be two ordinary friends who have just decided that they need a change in their lives. Maybe they've grown up together in a small English town and now want to see what the world outside England has to offer.
One possible reason could be to protect Teddy. Maybe there are some threats in England that Harry wants to keep Teddy away from. For example, there could be some dark wizards or dangerous magical creatures that target Teddy for some reason.
It's clear that Harry and Daphne are the main characters. Harry brings his courage, skills, and a network of friends (even if they're leaving England). Daphne may bring a different perspective, perhaps she has a more academic or cultural knowledge that will help them in the new places they'll visit. Their relationship and how they interact during this departure from England will likely be a major focus of the fanfiction.
One possibility is that they leave England in search of a magical place that can offer them a peaceful life. They could be tired of the constant battles and power struggles in England. They might go to a far - off land where magic is used in a different, more harmonious way. For example, they could head to some hidden island where ancient magic thrives and they can study and grow without the interference of the Ministry or dark wizards.
It could be that Harry and Susan are escaping some sort of danger in England. They leave secretly and head to France. There, they try to blend in with the local community. Harry might find work as a painter and Susan could start teaching English. Their journey would be about not only starting anew but also constantly looking over their shoulders in case the danger from England follows them.
Perhaps they would head to America. In the wizarding world of America, there are different magical institutions and a large expanse of land where they could start anew. They might be drawn to the Ilvermorny School of Witchcraft and Wizardry area, to interact with new wizards and witches and learn from a different magical education system. Also, the American magical community might not be as involved in the affairs of the English wizarding world, providing them with some anonymity.
One possible reason could be to escape the danger and chaos that might be brewing in England within the wizarding world. Maybe there are dark forces targeting them specifically, and leaving is their only option for safety.
One interesting fanfic could be 'A New Start'. In it, Harry and Hermione leave England due to the constant reminders of the war and a desire for a fresh start. They travel to a small, idyllic village in France where they begin to explore new forms of magic and build a life away from the prying eyes of the British wizarding world.
One reason could be to escape the trauma of the war. They've been through so much in England, and leaving gives them a chance to heal in a new environment.