While you make a lot of good points, I don't think you're addressing the impact of the POD, which posits additional military forces permanently assigned to Britain, and the subjugation of a lot of the forces that later helped to tear down Roman Britain. While being an island doesn't make Britain immune from invasion, it certainly helps, and I'm not sure you can say that it would ever fall. The Byzantines survived way worse threats (way, way, way worse threats) than Roman Britain did - why not an enduring Romano-British polity?