Clan Badges

[MacMillan of Rathdown]

Is there a clan crest with a decapitated head representing a Campbell

Yes there is. Chisholm quite clearly depicts the sacred boar head of the Campbells on a dagger. That's the Highland equivalent of putting the Profit Mohammed's head on a stick. Far more insulting than using the old stand-by "Head of A Savage" which is how warring clans referred to each other, and why that charge appears on the odd Scottish crest.

Or an Englishman?

I suspect you haven't seen the crest of the Adairs? The head of a nice beared Englishman, complete with blood dripping from the neck. You might also look at the arms of Lloyd, as well as several other Welsh families who display Saxon Heads as part of their armorial achievement. Fauntleroy uses the head of an infant, although to be fair this is an example of canting arms, the origin of the name being "l'enfante la roy"

If not, why do you think there isn't?

Well. Let me put it this way. I do not wish to sound vexatious when I suggest that it seems to me you may have an ulterior motive for raising the question about the ethnicity of heads found as heraldic charges, usually in clan badges. Whether this is to cast aspersions upon the British, or to raise issues dealing with some sort of wrongly perceived unfair racial attitudes in heraldry I do not know. Nor do I really care. I have thus far answered your questions, and would prefer that discussions on heraldic matters remain just that-- focussed on heraldry and not wander off into the arena of "social grievance". That said, I'll answer your last question:

Sloane Evans's The Grammar of Heraldry lists no fewer than eight heraldic heads: (1) Wild Man's or Savage's); (2) Moor's; (3) Saracen's; (4) Saxon's; (5) Englishman's; (6) Old Man's; (7) Woman's; (8) Child's.

As I've shown, with the exception of canting arms and crests (Moor punning Muir for example), Saxon's, Savages, and Englishmen seem to outnumber Moors and Saracens in British heraldry by a fair old number.

Fewer than about 5% of crests encountered in British heraldry are representational of the human head. Why is that? Well, I believe it is down to two things:

First, heads, even skewered on a sword, just don't look fierce or ferocious. There is a reason that the most common charge is a lion-- it's ferocious and it looks cool. Even poorly drawn it is still a lion, with all that it conveys.

Second, people in the British Isles regularly intermarry, and nothing is more apt to put the kibosh on a marriage than showing up at the castle to ask for the bride's hand when you have her Uncle Bulgaria's head being used as the crest on your helmet. No matter how much land you may have, the wedding is probably off.

I hope that answers you questions.