Queens and Kings of Great Britain have never been called Empresses/Emperors. Was it related to the fact that Great Britain (and its dominions) was a de jure kindgom, instead of an empire, as claimed by the designation "British Empire"?
Could you define what you mean by 'de jure kingdom' and 'de jure empire' in a non-CK2 context?
The English monarchs were in fact formally "emperors" after the creation of the "Empress of India" title in 1876, held by every monarch from Victoria through George VI.