It is noteworthy that the Dumnonii were behind the British peoples in the east. They had no coinage, whereas those in the south-east had theirs, copied from a stater of Philip of Macedon. No Greek coins had reached the Dumnonii, and they had consequently none to copy, however clumsily.
In the battle of Deorham, 577, the Britons were defeated with great slaughter, and the West Welsh of Devon and Cornwall were cut off from further communication with their brethren of Wales. The Saxons steadily advanced, but for long the Parrett was the boundary. In 823 a battle was fought between the Saxons and Britons at Gavulford, now Galford, a point on the old road from Exeter to the west, where the hills draw together, and whence it is commanded by a huge camp. The Britons now called the Danes to their aid, and twelve years afterwards a battle was fought on Hingeston Down, above Calstock, in which Egbert was victorious. This was in 835. Hitherto the Britons had occupied one portion of Exeter, but Athelstan, after defeating the Cornish King Howell, not only expelled them from the city but fixed the Tamar as their boundary. Then he passed through Cornwall, and even visited the Scilly Isles. The Count of Poher in Brittany, of whose son Alan Barbetorte was the godson of Athelstan, fled from Brittany with a crowd of his countrymen from the devastations of the Northmen, and Athelstan gave them homes in England. It is probable that he planted some in the Lizard, and others about Camborne, for we find there church dedications to distinctively Breton saints, and we know, moreover, that the fugitive Bretons brought with them the bones of their patron saints. As they spoke the same tongue as the Cornish, it would be natural for Athelstan to send them there.
In 981, the Danish pirates plundered the monastery of St Petrock, and in 997 ravaged the territories of their old allies from one end of Cornwall to the other with fire and sword. Shortly before this, in 993, Olaf Tryggvason of Norway, with Sweyn Forkbeard of Denmark, after having attacked London, devastated the east coast, burnt Sandwich and Ipswich, and stormed Bamborough, then harried the Scottish coast, the Western Isles, the Isle of Man, and Ireland, where "he burned far and wide, wherever inhabited." He then came to the Scilly Isles, where he put into Tresco harbour. There the monks of the abbey founded by Athelstan so impressed him, that he consented to be baptized.