Charlie Higson 
The Fear (The Enemy Book 3) 

HE DOESN»T KNOW IT BUT DOGNUT IS ABOUT TO SET OFF A CHAIN OF EVENTS THAT WILL AFFECT EVERY KID IN THE CITY.
The sickness struck everyone over the age of fourteen.
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 . . .they»re hungry. They»re bloodthirsty. And they aren»t giving up.
Dog Nut and the rest of his crew want to find their lost friends, and set off on a deadly mission from the Tower of London to Buckingham Palace and beyond, as the sickos lie in wait. But who are their friends and who is the enemy in this changed world?