I tend to feel that England, Wales, Scotland, Ireland is the maximum feasible division of the Isles to result over an extended period of history. And Wales is dicey due to geography, the Welsh mountains being no real shield as so many valleys open into England, and historic Welsh disunity; they would have had to get their act together big time to fend off English dominion for longer than they did, but as it was they spent more time fighting each other than the English. Maybe if matters had gone differently with Llewellyn the Last there could have been an autonomous Wales keeping its own laws (which it did anyway until Henry VIII abolished them), with a Prince owing feudal duty to the King of England.
I don't feel Cornwall is a chance, too small and isolated from any possible allies. It was absorbed into Wessex so long ago that we aren't sure of exactly how or when, and know little of its independent history. Speaking of Wessex, it is perfectly true that English union resulted directly from the Danish invasions, with Wessex as the last man standing taking the prize once the Danes had been defeated. Absent the invasions, and Danish successes, union could have taken a lot longer, and never come about in exactly the same way, for example with the northernmost counties winding up in Scotland, where they were part of the time anyway.
Union between England and Scotland was never inevitable. The English more or less gave up efforts at conquest after Bannockburn, Mortimer and Isabella formally acknowledging Scottish sovereignty under Bruce in the 1328 Treaty of Northampton. The underage Edward III fiercely resented the concession made in his name, and after his successful coup tried doing something about it, but he had too many other fish to fry for an extended effort. I think it would be right to say that despite numerous conflicts the English never seriously set about conquering Scotland again, and of course in the end the union was accomplished by the Scottish king inheriting England. All kinds of dynastic divergences might have prevented that.
A possible post-Heptarchy break-up of England would have been on north-south lines. The north suffered worst from the Conquest, William I laying utter waste to it. Recovery was slow and in later centuries northerners were still seen as barbarians by southerners, and returned the dislike with interest. There were many reasons for Richard III's profound unpopularity, the child murder of which he was universally and no doubt correctly suspected being foremost, but his northern entourage and troops were another and cogent reason for his lack of acceptance in the south. Conversely, in the north he was seen as their man and loyally supported despite all his crimes.
So the Tudors were never loved in the north, and the English Reformation was also far less accepted there than elsewhere, as witness the Pilgrimage of Grace. Elizabeth I, the last and longest-reigning Tudor, never in her life went north of Nottingham, her Council feeling it was not safe for her (there was a proposal which nearly came to fruition for her to have a summit at York with the still-reigning Mary I of Scots, but it was cancelled due to events in France). So somewhere in there a northern breakaway might be thought feasible, probably after a lengthy and stalemated dynastic/civil war.