Thanks, I had realised that but the legal situation is a little more complex. According to a recent letter from Anthony Joseph Emeritus President Jewish Genealogical Society of Great Britain in Family Tree an Uncle /Niece marriage is allowable under Jewish Law; but NOT an Aunt/Nephew marriage. Though an Uncle/Niece marriage is illegal under English Law, if the marriage has taken place overseas the couple can live together in the UK lawfully as husband and wife.
That is one situation clearly shown, i would expect there are others emanating from other faiths.