Note that the I haplogroup centers in the Balkans about 12000 years ago, and migrates north as the ice recedes. This haplogroup is associated now with northwestern Germans, Dutch, and the Scandinavians, as well as Anglo-Saxon English. The Celts, by contrast, are represented to some degree by the R1b haplotype, who apparently took refuge in present day Spain during the Ice Ages, and spread back north about the same time. Eventually the two groups mixed in France and Britain.
But it seems apparent that Southeastern Europe used to have a lot of people we would now think of as belonging only to Northern Europe.
The below link is a DNA map by professor Stephen Oppenheimer, you'll like it.