I am also 100% British Isles and have some concerns, especially as that seems to mean heavily Celtic. I have broadened my understanding of Celtic and I do see more than I might have first thought. However, my ancestors were -not- all Celtic. I should have a small amount of French and Prussian but even if that fell into a broader understanding of early Celtic it is still extremely unlikely that my ancestors were that isolated from the entire Scandinavian group.
It has been said that people can have matches whose ancestry does not share the same ethnicity.
A separate question is can we have ancestors that are from ethnic groups not indicated? If not then I find this very difficult to work with and unlikely. No one is that isolated as to end up as 100%.
If it is that the ethnicity did not show up in my particular mix (not everything gets passed down) than that would make more sense. Or is it possible that there will simply be some refining as ancestry progresses? Or that things may change after the test is given to those outside of the U.S?
It is actually encouraging to me that others have run into similar concerns. It is also encouraging that ancestry has already indicated that there may be some changes in the ethnicity as more people test.