Kate Nash, a 20-year-old London singer/songwriter, made her initial impressions, like Lilly Allen, through MySpace, and her breakthrough single, “Foundations,” is a brittle, sunny-sounding jingle—similar in tone and spirit to Allen’s kiss-off “Smile”—about a futile teenage romance that OMG! has the words “bitch” and “shit” in it. (There’s a sultry little number called “Dickhead,” too, and a synth-pop ditty called “Shit Song.”) But Nash isn’t really that similar to Allen at all—her piano-based songwriting leans toward rock and folk rather than radio dance-pop. She’s like a less studied Nellie McKay, or a public-school version of Alanis Morissette, 13 years on. She has a distinct proclivity for adolescent shock, but she’s an engaging, if not overwhelming, singer, even if her affected Cockney accent has earned some criticism. She displays an ear for a hook (“Skeleton Song,” “Foundations”) and her lyrics are sometimes witty, minute observations, though they rarely add up to much (“You’ve gone and got sick on my trainers/I only got these yesterday/Oh, my gosh, I cannot be bothered with this,” from “Foundations”). But she should be careful: Instant celebrity can be a bitch, especially when it’s bought with a smarty-pants potty mouth.