The Battle of Deorham occurred in 577 between the West Saxons and the Celts. Deorham is usually taken to refer to Dyrham, Wiltshire. The battle is considered by some to be decisive since it drove a land wedge between the Celts of what is now (approximately) Wales and those occupying territory to the south-west, e.g. what are (approximately) now Somerset, Devon and Cornwall.
However, passage through this area by Welsh-speaking individuals was not impossible: a genealogy of Welsh aristocrats notes that the descendants of kings who ruled in Powys founded a dynasty of rulers in the Glastonbury region in the eighth century.