Great Britain and its global holdings during their golden years is often referred to as 'The British Empire' despite being ruled by a regular old monarchy. Were there any monarchs who tried to fancy themselves as the 'British Emperor?'

by bigdoublek96
Noodleboom

More can certainly be said, particularly about rulers preceding Victoria, but here's some jumping-off points:

I discussed how Victoria came to be styled Empress of India while remaining Queen of other dominions here.

/u/Temponautics and /u/Dongzhou3Kingdoms discuss the general political and ideological concerns monarchies and titled nobility have to weigh when laying claim to a title here and here respectively.