From Wikipedia

Dark Age Britain

Sub-Roman Britain is a term derived from an archaeological label for the material culture of Britain in Late Antiquity: the term "Sub-Roman" was invented to describe the potsherds in sites of the 5th century and the 6th century, initially with an implication of decay of locally-made wares from a higher standard under the Roman Empire. It is now often used to denote a period of history. Although the culture of Britain in the period was mainly derived from Roman and Celtic sources, there were also Saxons in the area, originally from Saxony in north-western Germany, although the term 'Saxon' was used by the British for all Germanic incomers. Gradually the latter assumed more control (see Anglo-Saxon England). The Picts in northern Scotland were also outside the applicable area.