>>Finally, the isles were conquered by Normans — a third Germanic wave.
the Normans were culturally French, not German.
The Normans were culturally French and spoke French in 1066, but had some "Northman" (Norwegian) ancestry. Parts of England were conquered by the Danes--it was called the Danelaw. A number of basic English words are of Scandinavian rather than Anglo-Saxon origin as a result (they/their/them, skull, skirt, egg and others).
Parts of Scotland were under Norse rule during the Middle Ages.
The Celts were not the earliest inhabitants. Stonehenge was built by a pre-Celtic population.