There are a lot of defining mutations for I1 suggesting a serious bottleneck before expanding.
Haplo I is supposed to have survived LGM in European refuges with subsequent northward expansion.

Just a tought : could pre-I1 have gone north towards Doggerland after the LGM?
The flooding of Doggerland would have killed a lot of them leaving just a few survivors.
Then these survivors still had to find a piece of land for themselves.

http://my.opera.com/nielsol/blog/201...ded-doggerland

The survivors might have found a small piece of land somewhere in Danmark or in some fjord in southern Norway.
Or maybe even a small island that didn't drown.

The present distribution of I1 could be explained as the result of expansions during bronze and iron ages.