I wrote about this before: https://www.reddit.com/r/AskHistorians/comments/3ptbu7/why_isnt_the_queen_of_england_titled_empress/

The tl;dr (there are several answers in there) is that the British monarchs did have a title of Emperor/Empress of India as part of their styles, but "empire" in English usually means a multinational/multiethnic/multinguistic state (which the Raj certainly was). The political entity of Great Britain itself was not considered such at the time, and in any case decolonization means that the current queen is individually the queen of many of the commonwealth states, but not of a unified political entity comprised of those states.