"a british dual nationality = 100% a british citizen, not half of one! It's not like "british subject" which isn't a full citizen. Where is this heading? scary?"
YY
I agree with Xiang and Sandy.
and
"The implication is that dual nationals are lesser citizens if they are being counted in a different group and not counted in the "British citizen" column."
But of course it is. Information about English as second language is sort of relevant I think but why are they interested in who has dual nationality??
You could spin this further and say that children with dual nationality are seen as not properly British? It's too much information to ask.
Can you all see where this is going to go? In the near future, people who become naturalised will probably not be allowed to retain their other citizenship. Or, at the age of 18 the child has to decide to be either British or the other nationality.
Would make Brexiters happy, probably.