Expendables by Alison Ruth Ingleby

One girl. A corrupt government. A secret that could shatter a country.
London is divided by a lethal barrier. Inside the Wall, the rich live in wealth and privilege, supported by the corrupt government. Outside the Wall, people starve, and children are taken by the government—never to be seen again.
Twelve years ago, Aleesha narrowly escaped capture when her mother disappeared. She survives on her wits, trusting no one. But when a rebel organization called the Chain offers her the opportunity to uncover her mother’s fate and her father’s identity, she jumps at the chance. After all, she has nothing left to lose.
When the Chain forces Aleesha to team up with Trey, Aleesha is furious. The only thing worse than being sent on an impossible mission is having to drag along a rich kid with zero street smarts. But when events take a devastating turn, Trey may be the only one who can save her.