Yes, most participants are American, searching for their British roots. However, some can be traced back to Yorkshire via Ireland, and one Yorkshireman comes from East Riding (his deep yDNA roots actually match those of Thomas Jefferson, who likely traces back to Yorkshire via Snowdonia; their common ancestor is pre-1700). I don't know of any British-based Kidd DNA study and, indeed, the American's would be ecstatic if one was started as they could then stand a better chance of finding geographical localities where their roots lie.