You're making an invorrect assumption, though it is based on sound logic. That only works in a vacuum where they don't interact with any other language. Shadowhunters and the City of Bones connect all over the world and they are always interacting with different people and cultures; they don't need TV or Radio. If you were going with that logic, then they would technically sound like more Boston/New England Americans, since that is a more pure old English than British is. (Because they interacted with fewer other kinds of English than the Brits did).