Bloodtide - Melvin Burgess

Set in the future as two rival gangs, the Volsons and the Connors, struggle to control London. The city is almost cut off from the rest of the country and surrounded by the Halfman Lands - a desolate area populated by creatures, Halfmen, the resultsof genetic experiments to create living killing machines. Val Volson dreams of uniting the two factions by giving his 14-year-old daughter, Signy, as a bride to the rival ganglord, Connor. A shocking betrayal almost wipes out the Volsons - only Signy and her twin brother, Siggy, survive. The fate of both gangs, and of London, is in their hands, but both must pay a terrible price for power.