During the night of the "Fiesta de los Niños Santos" (or "Festival of the Saints' Children", as maid Catalina calls it in English), Paul (Hannah's father) tells Dr. Contreras Jr. that he and Sarah met in New York, then moved to London and got their daughter there, and have been living in London since, until moving to Colombia now. So apparently it's implied that Hannah grew up in London, and hence picked up a British accent from her environment, such as her classmates and teachers at school.
