The economic advantages, plus the traditional Nordic and Germanic habit of having free females own the house, wagon, farm, etc. while their husbands owned the more mobile wealth, would tend to discourage male intermarriage with slave women, who didn’t own anything useful and couldn’t give a free man a home. (I suspect that brawny male relatives discouraged free women from marrying slave men.)

However, given that Cheddar Man’s descendants still live in Cheddar, I’m thinking that “all the native Britons fled and faded away” is saying a bit much.