My main concerns with Romano-British timelines are as much about economics as they are political unity and strength.
Economically, Britain isn't The East. It isn't even Italia. But it is still a reasonably wealthy region that has good natural defences. What it does have is the potential for lots of coastal trade.
Politically, it's hard to keep it together, even when the Romans were strong. So you need to unite the people. Southern Britain has more Romans and Romano-British, so that's a fine base, but I doubt you can realistically expect to have them dominate the others without some co-operation. A good option could be to claim to be Bretwalda, and focus on economic domination rather than military force. But ensure you have a that title tie people's together with the Bretwalda only policing the peace, rather than forcing others to serve. (At first)
Strength, Britain was a naval power when it took off, it's immediate threats are naval, so the best way to solidify power, and move forces, would be to focus on that. Strong coastal forts, logistics and communication by sea, and dealing with raiders. That way a smaller force can be applied repeatedly and rapidly. This means forts in Ireland, Scotland, and likely Frisia, as well as the major rivers. The remainder focus on securing what would become the heart of Wessex in the south.
An alternative to all this is Coel Hen however. He was an alternative in the North. If you can somehow connect the two Generals in a timeline, that could be cool.
