Bloodtide by Melvin Burgess

London has been abandoned by the rest of the world. For generations now, London has been ruled by two warring gang lords, the Volsons, controlled by Val and his four children, and their rival, Conor. Now, Val plans to marry off his only daughter to Conor to form a treaty between the two. Val dreams that with the humans united, they will be able to break through the circle of halfman that surround London and move out into the world. At first, it seems that the treaty might actually work, but there's always the danger of betrayal...