Hmmm....interesting!
Approximately 13% of the UK population comes from Asia or Africa, so I can understand that they wouldn't have German ancestors. I wonder how far the ONS went back. Official records only exist dating from the sixteenth century, so it's difficult to go back any further. Did the ONS base their figure on DNA tests or paper records?
Those with Irish ancestors are quite interesting and possibly don't have German blood. Irish nationalists destroyed many records, so the English couldn't get their hands on them, but DNA tests show that many of them are descended from the tribes which inhabited the island of Great Britain before the Angles, Saxons, Jutes (and later Vikings and Normans) arrived, so they possibly have a claim to be more English than the English.
Some Welsh are descended from the original Britons and also have Spanish DNA from the days when the Spanish invaded. Scots from the East Coast have different DNA patterns from those on the West Coast. Those on the East coast are likely to have Danish DNA.
This is all nonsense anyway, because mobility has meant that most of us are mongrels.
I spent years researching my family history, so I have details of about 12 generations (over 8000 names). I was born with an unusual surname, which was easy to research, my paternal grandmother's family owned many acres of land, so there are legal records, and my mother's family all stayed in the same villages for centuries.
That's how I can claim to be English to the core, but I don't know (and will probably never know) where most of my sixteenth century ancestors came from. I have files of paper records, so I am legally English, but who knows whether any of my ancestors were born as a result of clandestine relationships?
It's probable that we all came from Africa. Countries and nationality are man-made concepts.