Ethnic background of the British Royal family

From the website of genealogist William Addams Reitwiesner, who died last year. The Ethnic ancestry of Prince William (b. 1982):

Every so often, someone will state that the British Royal Family is "not British", that they are instead "German" or "Foreign". Since this belief seems to be somewhat wide-spread, and since the genealogy of many members of the British Royal Family is fairly well known, it seemed to me that it would be fairly easy to quantify precisely how "British" or "non-British" the British Royal Family is. This webpage shows the results of my work.

[. . .] The definition of "ethnic group" I'm using, and the working definition most people seem to use when referring to someone's ethnicity, is that all of the known and/or traceable ancestors of the person in question were part of the indigenous population in a certain geographic location, where the inhabitants shared a common language, a common culture (including a common religion), and an exclusive mating population (i.e., the members of this group would only mate with other members of the same group). The person can then be considered to be a member of that "ethnic group". [. . .]

Another place where derived values can be adjusted is in my handling of one certain ethnic group. Long study of the genealogies of the European Royals can lead one (specifically, has led me) to the realization that the European Royals can be considered a separate ethnic group. The only way they differ from other ethnic groups is that they are not geographically discrete, but in other respects they meet all the qualifications of an ethnic group: their shared rituals, their shared language(s), etc., and (most significant from a genealogical / genetic perspective) their mating habits. Until fairly recently, the only acceptable mate for a Royal, male or female, was another Royal. Royal = non-Royal matings which resulted in Royal offspring are notable primarily for their scarcity. For this reason, I have described someone whose ancestry is exclusively or primarily from this Royal caste to be of the "Royal" ethnic group.

It's possible that you, the reader, may not wish to accept my definition of the "Royal" ethnic group, and would rather describe persons of the Royal caste by using one or another of the traditional ethnic group names. Feel free to do so. Since most of the "Royal" persons referred to below lived in what is now Germany, the most convenient traditional ethnic group name to refer to them would be "German". Simply add the values I refer to as "Royal" to the values I refer to as "German" and call the result "German". I, however, will maintain the distinction in the layouts below. [. . .]

Prince William of Wales:

35.327 148 437 5 % English
29.418 945 312 5 % Royal
14.404 296 875 % Scottish
5.383 300 781 25 % German
3.747 558 593 75 % Irish
3.320 312 5 % French
3.295 898 437 5 % Anglo-Irish
2.343 75 % Hungarian
0.781 25 % Armenian
0.585 937 5 % Dutch
0.292 968 75 % Danish
0.292 968 75 % Welsh
0.158 691 906 25 % Belgian
0.146 484 375 % Swedish
0.146 484 375 % Swiss
0.122 070 312 5 % Bohemian
0.097 656 25 % Lithuanian
0.097 656 25 % Russian
0.036 621 093 75 % Jersiaise
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100.0 %