Enemy series (Charlie Higson)

After a disease turns everyone over sixteen into brainless, decomposing, flesh-eating creatures, a group of teenagers leave their shelter and set out of a harrowing journey across London to the safe haven of Buckingham Palace.

The sickness struck everyone sixteen and over. Mothers and fathers, older brothers, sisters, and best friends. No one escaped its touch. And now children across London are being hunted by ferocious grown-ups who are hungry, bloodthirsty, and not giving up. DogNut and the rest of his crew, in search of the friends they lost during the fire, set off on a deadly mission from the Tower of London to Buckingham Palace and beyond, as the sickos lie in wait....

"First the sickness rotted the adults' minds. Then their bodies. Now they stalk the streets of London, hunting human flesh. The Holloway crew are survivors. They've fought their way across the city and made it to the Natural History Museum alive--just barely. But their fight will never end while the Enemy lives, unless there's another way... The kids at the museum are looking for a cure. All they need are medical supplies.To get them they must venture...

"The final battle ensues between Saint George's army of sickos and the army of London kids"--