The Enemy (The Enemy, #1)

In the wake of a devastating disease, everyone sixteen and older is either dead or a decomposing, brainless creature with a ravenous appetite for flesh. Teens have barricaded themselves in buildings throughout London and venture outside only when they need to scavenge for food. The group of kids living a Waitrose supermarket is beginning to run out of options. When a mysterious traveler arrives and offers them safe haven at Buckingham Palace, they begin a harrowing journey across London. But their fight is far from over-the threat from within the palace is as real as the one outside it.
Full of unexpected twists and quick-thinking heroes, "The Enemy" is a fast-paced, white-knuckle tale of survival in the face of unimaginable horror.