Complex sentence: They lived throughout Britain south of about the Firth of Forth; after the 5th century Britons also migrated to continental Europe, where they established the settlements of Britanny in France and the obscure Britonia in what is now Galicia, Spain.
Split into simpler sentences: They lived throughout Britain south of about the Firth of Forth. After the 5th century Britons also migrated to continental Europe, to the settlements of Britanny in France and the obscure Britonia in what is now Galicia, Spain.