What is the author's style in Sister: A Novel by Rosamund Lupton?

The author uses primarily a narrator, Bee to tell this story. So, both British slang and American slang are used to make Bee a more believable British woman who has lived and worked in the states for several years. The difference between British and American terms is heightened when Bee remembers Tess teasing her about her Americanisms and when others quote Tess incorrectly to Bee, because they use American terms rather than British, and Tess did not car for Americanisms.