Re: An Impossible Problem ???
What Haplotype? We are a mixture of everything here, and am not sure we Croats have a separate one – how do you know your male ancestor was a Croat?
Closest hit for Nichols would be Nikoli? (but as you said it’s a long shot).
I think: the way to go is focusing to his parish/town in England and sources there (church, registers etc) to see if his origin was stated anywhere. All this IF he was relatively recent immigrant (a few centuries back).