Unless the Scots control southern England they are not going to dominate the British Isles because they don't have the resources. The reason for this is that the best agricultural land is there and so locals can raise larger armies. Victory in a battle or two is not going to be enough because the dwellers in that area whether Catuvellaunian, Roman, English or Norman will eventually drive the Picts/Scot back north.
Lots of nice agriculture doesn't do very good when it isn't unified. In fact, it just starts to look tasty. If divided, which the Anglo-Saxon kingdoms were for a very long time, against a unified Norse-Celtic front, something that rarely happened but did at Brunanburh, you have a very different situation.
Should the Scots/Norse/Brits/Picts win at this battle, it is an
opportunity (arguably the last real one too) to check the Anglo-Saxon advance. Should the allied army against the Anglo-Saxons succeed at this moment, preferably with Athelstan's death in battle, you would see York return to Norse control (if not claimed by the Scots themselves), and probably a divided England between Mercia and Wessex, not to mention the armies marauding southward. You also have a stronger British/Welsh position in centuries to come.
A politically divided England is a weak England, especially against a united Scotland. And we're talking about the 900s AD, the height of the Viking Age. You don't think that at the first sign of weakness from England, a Danish army isn't going to make for those wonderful farmlands you were talking about?