In an earlier post on Intermarriage I discussed some examples of marriages between Brittonic and early English ('Anglo-Saxon') royalty. The presence of Brittonic names in Anglo-Saxon genealogies, and a possible Brittonic warrior whose father had an Old English name, may be further supporting evidence for intermarriage.
The genealogy of the kings of Lindsey (roughly modern Lincolnshire, see map [http://www.carlanayland.org/exile/map_britain.htm]) is given in the Anglian Collection:
Woden; Winta; Cretta; Cwedgils; Caedbaed; Bubba; Beda; Biscop; Eanferth; Eata; Aldfrith
--Anglian Collection, available online (scroll down)
None of the individuals can be securely dated. Bede mentions a man called Blaecca, who was the reeve of the city of Lincoln in around 628 (Book II ch.16). If this Blaecca was some sort of relative of the three kings beginning with B- in the genealogy, as might be consistent with the habit of alliterative naming and his possession of a position of responsibility, then those kings might be tentatively dated to somewhere around the early to mid seventh century, but this really is clutching at straws.
For the purposes of the current discussion, the name of most interest is the one immediately preceding the three B- kings, Caedbaed. This name contains the common Brittonic name element Caed- (also spelled Cat- or Cad-), which derives from the word for 'battle' and occurs in the names of numerous documented Brittonic kings and princes in the seventh century, including Cadfan, Cadwallon and Cadwallader of Gwynedd (see earlier post on the Chronology of the Kings of Gwynedd]) and Cadafael Catguommed (see earlier post on Cadafael). Does its presence in the genealogy of the kings of the Anglian kingdom of Lindsey indicate a coincidence, a fashion in names, a scribe who mistakenly copied the name in from somewhere else, or a dynastic connection with Brittonic royalty?
Bishop Asser, writing in the late ninth century, gives the genealogy of Alfred the Great as follows:
King Alfred was the son of king Ethelwulf, who was the son of Egbert, who was the son of Elmund, was the son of Eafa, who was the son of Eoppa, who the son of Ingild. Ingild, and Ina, the famous king of the West-Saxons, were two brothers. Ina went to Rome, and there ending this life honourably, entered the heavenly kingdom, to reign there for ever with Christ. Ingild and Ina were the sons of Coenred, who was the son of Ceolwald, who was the son of Cudam, who was the son of Cuthwin, who was the son of Ceawlin, who was the son of Cynric, who was the son of Creoda, who was the son of Cerdic, who was the son of Elesa, who was the son of Gewis, from whom the Britons name all that nation Gegwis
--Asser, Life of Alfred, available online
The Anglo-Saxon Chronicle says:
A.D. 495. This year came two leaders into Britain, Cerdic and Cynric his son, with five ships, at a place that is called Cerdic's-ore. And they fought with the Welsh the same day. Then he died, and his son Cynric succeeded to the government, and held it six and twenty winters
A.D. 519. This year Cerdic and Cynric undertook the government of the West-Saxons; the same year they fought with the Britons at
a place now called Charford. From that day have reigned the children of the West-Saxon kings.
A.D. 534. This year died Cerdic, the first king of the West-Saxons. Cynric his son succeeded to the government, and reigned afterwards twenty-six winters.
--Anglo-Saxon Chronicle, available online
Never mind the contradictory dates; there is clearly a tradition that an important early king of the West Saxons was a man called Cerdic. This is the same name as the Brittonic name Ceretic or Ceredig. Bede mentions a Brittonic king Cerdic (Book IV ch. 23), at whose court St Hild of Whitby was born in around 614 (probably the same Ceredig whose death is recorded in Annales Cambriae in 616).
616 Ceredig died.
-- Annales Cambriae, available online
At least one later king of the West Saxons also had a Brittonic name. Bede describes a king called Cadwalla (the same as the Brittonic name Cadwallon, see above under Caedbaed of Lindsey) who made himself king of the West Saxons by military force in around 686 and died on a pilgrimage to Rome in 689 (Book IV ch. 16; Book V ch. 7). Bede explicitly says that he was a member of the West Saxon royal dynasty (Book IV ch. 5).
So the West Saxon dynasty was founded by a man with a Brittonic name, and a member of the same dynasty also had a Brittonic name in the late seventh century. This could be coincidence, fashion or may indicate a dynastic connection with Brittonic royalty.
Y Gododdin is a Brittonic epic poem describing a disastrous attack by a warband from Gododdin (roughly the area of modern Lothian and Edinburgh) on 'Catraeth' (location unknown, possibly the Roman fort at Catterick in North Yorkshire). The date is unknown, but usually placed in the late sixth or early seventh century, although the poem survives only in a much later (around 13th century) manuscript. It mainly comprises elegies for fallen warriors. One of them, Yrfai or Uruei, had a father whose name was Golistan or Uolstan:
It was usual for Uolstan's son - though his father was no sovereign lord - that what he said was heeded
It was usual for the sake of the mountain court that shields be broken through reddened before Yrfai Lord of Eidyn
--Translation and reconstructed text by John Koch (stanza B2.28)
John Koch interprets Golistan or Uolstan as a form of the common Old English name Wulfstan (Koch 1997). Note that John Koch's interpretation of the historical context of the poem and battle is controversial, but the name Golistan/Uolstan doesn't depend on his theory about the historical context. If correct, perhaps this Wulfstan was a mercenary or exile in Gododdin ("no sovereign lord") who married his employer's daughter and whose son held a high rank in Gododdin's warband.
There are two reasonably well-documented inter-ethnic royal marriages from Northumbria in the early seventh century, with possibly a third from the same region in the late sixth century (see earlier post on Intermarriage).
Recognisably Brittonic names appear in the genealogies of the Anglian kings of Lindsey (Caedbaed, undated, possibly early seventh century) and the West Saxon royal house (Cerdic, possibly legendary founder, late fifth century; Cadwalla, late seventh century). There may be a hint of an Old English name in the patronym of a Brittonic hero in Y Gododdin (late sixth or early seventh century). Cross-ethnic naming may be merely a matter of fashion, or could indicate inter-ethnic dynastic connections.
I would interpret the documented marriages and the presence of cross-ethnic names to indicate that inter-ethnic aristocratic marriage could occur in early medieval Britain. There is insufficient evidence to say whether it was rare or widespread, or how its occurrence may have varied by region or over time.
Anglo-Saxon Chronicle, available online
Annales Cambriae, available online
Anglian Collection, available online
Asser, Life of Alfred, available online
Bede, Ecclesiastical History of the English People. Translated by Leo Sherley-Price. Penguin Classics, 1990, ISBN 0-14-044565-X.
Koch JT. The Gododdin of Aneirin. Text and context from Dark-Age North Britain. University of Wales Press, 1997, ISBN 0-7083-1374-4.