Are Scottish people an "ethnic group"?

Recently filled out a form.

It had an ethnicity section.

Options were black, Asian, etc.

And under white it had "White British", "white Irish", "white other European", "white other".

I thought "where's the white Scottish box?"

If "white Irish" is an ethnic group, how is it that "White Scottish" or "white English" or "white Welsh" or "white Cornish" are not?

I filled out "white Irish" as my mother's parents are both Irish but actually I feel myself to be "white Scottish" I suppose. Ethnicity is all a bit weird and confusing (is it culture, language, upbringng, birth, citizenship, passports, length of time spent in one country, religion, ancestors, Blood, self identity, or what?) and we're all humans I suppose.

I'm just curious what people think? How can "white Irish" be an ethnic group if "White Scottish" or "white English" or "white Welsh" or "white Cornish" are not ethnic groups?

Cheers the now pals