Despite the country as a whole often refered to as "empire" they are always mentioned as kings/queens. I know that some of them used title of Emperor of India but why not Emperor of Britain?
This question has already been asked and answered in this thread here
edit: Forgot to cite specific people. There are a number of good answers, but I personally liked /u/WhenTheRvlutionComes and /u/intangible-tangerine