I would highly recommend The Inheritance of Rome: Illuminating the Dark Ages, 400-1000 by Chris Wickham, an Oxford historian. Though it obviously deals with all of the territories of the Roman Empire, he goes pretty full-on in his explanation of Anglo-Saxon political structures in the wake of the collapse. He is trying to find continuity between Rome's collapse and the polities that sprung up in its wake. He makes the claim that Roman influence only gradually seeped out and that by 1000 AD, Rome's influence had finally bled away. He even goes into how Carolingian political structures influenced the Anglo-Saxons as well. It's pretty dense stuff, but fascinating if you give it a go.