Thank you, xhavnak.
Also, though you're right in Cornwall its the strongest, the old kingdom of Dumnovia included Devon. I can see it even with Dorset and/or Somerset, given either weak neighbours or strong allies. Norman England might even keep it large to take from Saxon England.
Under most circumstances, a strong alliance with Wales would be a good bet.
Another suggestion. A separate Danelaw, perhaps allied with Scandinavia against expansionist Wessex? It might even become a client of Scotland, several centuries down the line.