I found this interesting article on the internet which deals with the decline of Roman Britain, the encroachment of the Picts, Scots and Saxons on "Welsh" lands from Strathclyde to Kent.
Right: Britain circa 600AD. See how the 'Welsh' (Britons) Stretch from Cornwall, through modern Wales, Lancashire, Cumbria to Southern Scotland.
It's interesting that the writer of the piece shows how a Saxon cleric says that the sins of the English led to the scourge of the Danes on their lands, just as the sins of the Britons/Welsh led to the scourge of the Saxons; in effect that the failings of the Catholic inhabitants of these isles delivered them to the assaults of vicious pagans.
Students of our Welsh and Catholic past will find this article of great interest:
Roman Britain and the Saxon Advent