Tom,
Good question, but think about what was happening at the time the Aglo-saxons et al moved into Britain (450 - 600 I forget exactly when).
First, there were no early french, there were late romans (Already in Britain) and the Franks, who were german (they didn't adopt French until sometime after 800 a.d., long after England was settled by the Anglo Saxons.)
For the Franks to have moved in , they wuold have had to been a sea people, which they weren't & The Late Romans were having troubles of their own with groups like the Goths and the Franks.
Now if your asking what would have happened if the Late Romans of France decided to move into britain sometime between 500 - 600 a.d. we might have had the opposite effect, Germans moving further down into france proper, with a latinate Britain.