Seeing as they were both regions that had experienced prolonged Roman rule and Romanization at least in some areas, I don't understand why Gaul became the Romance language France, but Britannia was overtaken by the Germanic Anglo-Saxon.
There's definitely more to be said in reply to this question, especially regarding Gaul/France, but there's an excellent, very detailed post by /u/alriclofgar on what we know about the migration of Germanic peoples to Britain in this era which you might find very interesting.
There's definitely more to be said in reply to this question, especially regarding Gaul/France, but there's an excellent, very detailed post by /u/alriclofgar on what we know about the migration of Germanic peoples to Britain in this era which you might find very interesting.