Hmm.. Or maybe it was the Latin part. The Brits were probably at least as much Latin as they were Celtic by the time the Angles and Saxons came along. By the time the Normans made it over they were totally mutts. :)
Ya, it has been written before that the Latin did them in, genetically speaking, and well culturally. They lost their Teutonic virtu. Hitler probably read some of that stuff.