Doggerland

Most of the R1b1c9 with British Isles ancestry is thought to reflect "invader" deep ancestry, meaning descended from Germanic or Scandinavian invaders of the British Isles, either after the Romans left or possibly in a more pre-historic time.

Some even theorize that S21 arose in Doggerland, which was the area of what's now the North Sea that was a landbridge between the continent and the British Isles during the Ice Age.

Of course, since that area is now under the North Sea, it's virtually impossible to prove.