When it came time to invade what had become England, William took with him an army composed mostly of men from the Norman and Bretagne coasts, and down to Anjou. That is, his army was made up of folks who were primarily descendants of the old Briton refugees who came to Brezh in the 6th century. His very first "Norman" town was called Vauxhal, which is simply the way a French speaker (William) would spell the Gallo name of the county seat of Normandy at the time, which is, itself, a short form for "Carvajal".
This is undoubtedly the source of the Briton input among the Danish and Dutch populations that nowadays dominate so much of the life of the British Isles.
No doubt the Anglo-Saxon heathens put as many Britons to the sword and spit as they could. We should all be happy William put a stop to that nonsense.
William abolished the death penalty in England in 1088.
The Normans WERE Celts. Rollo was from Norway and brought many Danes with him to threaten the simple French king into giving them "Normandy". By the time William got around to invading, many of his men had married other Celtic French women. The battles of 1066 were largely battles between Scandinavians to see who would KEEP England. (The Scandinavians won. ---ggg---
>We can be sure of that because they held on to their Gallo language!
Actually, good linguistic researchers say that language is a very poor way to try to draw conclusions, because the languages do not remain, any more than our ancestors languages remained with us several generations after they came to America.