If you assume those that identify themselves as American, United States, or White are mostly people of pre-1800 British Isles origin, and that about half of those who self-identified as Canadians had similar origins, then there are 92.2 million Americans who claim British Isles origin out of 225.3 million Americans who identified their national origin, 40.9% of those who identified their origin are English, Scots, Welsh, Scots-Irish, or Irish by ancestry. (The assumption that the self-identified Americans are mostly of British descent is supported by the concentration of these people in Appalachia, the Upland South, the Ozarks, and North and West Texas, where (except for Texas), there is little black or Hispanic population and little impact by the European immigration of the 1840-1920 period.)
Out of a total U.S. population of 281.4 million as of the 2000 census, about 115.1 million Americans have roots in the British Isles, assuming those that did not identify their national origin were equally distributed among the population.
This sounds semsible - although I also know some German, Scandinavian or Dutch descents also use the general "American" label in the census and so the figure could be a little lower?
It looks quite astonishing when comparing the set of data between New Zealand and the United States: overwhelming of New Zealand Europeans have majority British/Irish ancestry, while it is not so for American whites.
Actually, many of those specifying Canadian ancestry are from Quebec, of French background, who came to work in the textile mills in New England.
We are not as Anglo as you think.