what is this novel's style?is it formal or informal or anything else?

Great Expectations is a first-person narration, which gives it a certain informality. That being said, at the time he is narrating, Pip has gone through his education and so is able to speak/write quite intelligently, which provides a formal sheen. While Dickens's language might suggest this is an extremely formal style, at its best Great Expectations is very much in the voice of its character.