Shrines to Breton saints are found in many regions of northern Europe.

St Albinus (St Aubin) of Vannes, Bishop of Angers, is honoured all across northern France and as far away as Poland.

St Samson is honoured in Conteville, of which Count Robert of Mortain’s and Bishop Odo’s father Herluin was made Viscount.

“Viscount Robert and his brother Odo” occur as witnesses to a charter issued by Count Eudon, Duke of Brittany, at Rennes prior to 1050.

“Alan Rufus, son of Count Eudon” witnessed a charter by Yves de Bellême, Bishop of Séez, dated between 1047 and 1067.

Alan Rufus was captain of William’s palace guard in Normandy, and often appears near Bishop Odo on the Bayeux Tapestry, though they probably became unfriends over various actions of Odo’s later.

The sources are scattered, diverse and numerous, so they require time and patience to collate.

For the Romano-British presence in northern and central Gaul in last years of the Western Roman Empire, the sources are two letters by Sidonius Apollinaris, Jordanes’ “Getica”, and the writings of Gregory of Tours and of Cassiodorus.

Sidonius was a Gallo-Roman bishop and senator who was contemporary with the events and a friend of the British leader he calls Riothamus, as well as being a friend of the Gallo-Roman official Arvandus who committed treason with King Euric of the Visigoths against Emperor Anthemius with whom Riothamus was allied.

According to the above sources, Riothamus sailed up the Loire with 12,000 men, established a base at Bourges, marched to Déols, was ambushed there by Euric, battled for hours, faced defeat, gathered as many men as he could and retreated into Burgundy. (The nearest ancient Burgundian town is Avallon, on elevated land protected by a tight river bend: see Google Maps.)

Jordanes’ “Getica” mentions the Armoricans among Aëtius’s allies against Attila in 451. The Armoricans are the inhabitants of Brittany.

It’s possible that the ancestors of the Angevins/Plantagenets were present, as they originated as Gallo-Roman soldiers in western Armorica from which they were expelled in 383 and subsequently migrated to Rennes.

Here is a scholarly article on the Diocese of Britonia in Galicia: https://britonia.gal/wp-content/uploads/2022/04/Simon-Young-2003-The-Bishops-of-the-early-medieval-diocese-of-Britonia.pdf

See also: https://en.m.wikipedia.org/wiki/Britonia

Here’s a taster for the Albret/Labrit dynasty: https://www.britannica.com/topic/Albret-family

Two 10th century Counts of Ponthieu had the P-Celtic (identical to Brythonic) names Haelchod and Herluin.

Charter 24 in the Cartulary of the Abbey of Landevennec records Count Haelchod and his son Herleuuin as witnesses of donations on 10 April and 13 August 954.

The abbots of Landevennec had fled Lower Brittany and crossed all the way to Montreuil-sur-Mer where Count Haelchod sheltered them. While in Ponthieu, they built a shrine to Saint Winwaloe (Guénolé in French).

Two 10th century Counts of Ponthieu had the P-Celtic (identical to Brythonic) names Haelchod and Herluin.

Charter 24 in the Cartulary of the Abbey of Landevennec records Count Haelchod and his son Herleuuin as witnesses of donations on 10 April and 13 August 954.

The abbots of Landevennec had fled Lower Brittany and crossed all the way to Montreuil-sur-Mer where Count Haelchod sheltered them. While in Ponthieu, they built a shrine to Saint Winwaloe (Guénolé in French).