Several years ago I tried to find any Isgrig/Isgrigg families after the 1700s and came up with nothing. Now I see that there are several listed in UK census records, as you pointed out. I intend to start looking again and will post anything of interest to this forum.
The family of John Isgriggs (listed in the 1871 census) living in Middlesex is the one I will look into first, as this is the same area where our common ancestor William III is from.
My grandfather (Buest Isgrig) once said he had been told that the Isgrig line was Welsh. I have never been able to prove that, so it is interesting there there is also an Isgriggs family in Wales in 1871.