Responding to Alanabit's post regarding Cornishman Bob Fitzsimmonds, it appears that he was taken with his family, to New Zealand when aged about ten years old. However, I can find no reference to him becoming a citizen of that country, or of America, where he lived when winning the World Heavyweight Championship.
Evidently he held three world titles at different weights. He beat Jack Dempsey in 1891, for the World Middleweight title and Jim Corbett in 1897 for the World Heavyweight crown.In 1903 he won the new World Light-Heavyweight title. If Fitzsimmonds did not become a citizen of either New Zealand or America, surely that means he was an Englishman when he won those fights? Maybe somebody knows different?