Why do you have to be British to be in Harry Potter movies?

Answer 1

Anyone with a speaking part is going to have to speak with a British accent, other countries have their own wizard schools so the students at Hogwarts would be British, and they are not going to bring extras from Another Country when there are thousands of kids here at theater schools.

In an interview, Rowling explained how it wouldn't be very practical to put an American actor or other foreign actor when the character is British.

Even if she did allow it. It costs a lot of money to fly actors in and keep them living here. It would be cheaper for them to use British actors anyway.