Maybe in the fanfiction, Voldemort's return in a different way causes the wizarding government to spend all its funds on defense. They might have to buy expensive magical weapons, hire an army of aurors, and build fortified defenses. This over - expenditure could lead to bankruptcy not just of the wizarding part of Britain but also have a domino effect on the non - magical part as the two economies are likely connected in some ways. Also, the fear of Voldemort could stop all trade and tourism, further worsening the situation.
Well, perhaps in this fanfiction, Harry Potter accidentally casts a spell that has a domino effect on the magical economy of Britain. For example, he might mess up a powerful charm related to the wizarding currency, leading to a rapid devaluation. Or maybe he uncovers some dark secret about the Ministry of Magic's financial management that causes a mass panic and economic collapse.
It could be a corrupt wizard within the Ministry of Magic's financial department. He might be embezzling funds and using dark magic to cover his tracks. When Harry starts to dig into the strange financial situations in Magical Britain, this wizard tries to stop him by all means, which eventually leads to the bankruptcy as the situation gets out of control.
Well, perhaps Harry discovers some major financial mismanagement in Gringotts. Maybe he finds out that the goblins have been embezzling funds or making really bad investments. This could lead to a situation where the bank is on the verge of collapse and Harry, being the hero he is, decides to expose the truth and in the process, bankrupts the bank as all the secrets come to light.
The goblins themselves could be the main antagonists. They are in charge of Gringotts and if they are greedy and corrupt, they would oppose any attempts by Harry to expose their wrongdoings that could lead to the bank's bankruptcy. They might try to stop Harry by using their magical skills and the defenses of the bank.
Perhaps in the fanfiction, the Ministry of Magic in Britain is depicted as corrupt or ineffective. Harry Potter, being a hero who values justice, would be frustrated with such a system. He might then grow to hate Britain as a whole because of this broken system that's supposed to protect its magical citizens.
In some fanfictions, it might be through the over - expenditure on magical infrastructure related to Harry Potter. For example, building huge Hogwarts - like schools all over Britain could drain the economy. Also, the cost of magical creature conservation, which is highly emphasized in the Potter universe, could be exorbitant if implemented on a large scale.
In some fanfiction, the 'Harry Potter' world might be depicted as ruining Britain by changing its social structure too drastically. For example, if the wizarding world suddenly becomes fully exposed to the Muggle world in a fanfic, it could cause chaos. Muggles might panic at the existence of magic, and the established order in Britain, both in the wizarding and Muggle parts, could break down.
In certain fanfiction, the magical education system in 'Harry Potter' could have a negative impact on Britain. For example, if Hogwarts starts to produce poorly trained wizards due to a change in curriculum or teaching staff. These incompetent wizards might then go on to make mistakes in using magic in the real world, causing accidental damage to infrastructure and endangering lives in Britain.
In fanfiction, Harry could flee Britain due to a sense of disillusionment with the wizarding world there. He has been through so much trauma and has seen the flaws in the system. For example, the way the Ministry handled the threat of Voldemort in the past was often inept. He might also be seeking a place where he can explore magic in a different way, free from the traditional constraints of the British wizarding community. He may want to start anew in a place where no one knows him as 'The Boy Who Lived' and can just be Harry, an ordinary wizard exploring his magical abilities.