I have heard (please correct me if I'm wrong!) that at least one major motivation behind the creation of the 'Empress of India' title was so as not to be outdone by the Imperial titles of other great powers at the time. Why, then, was India chosen as the location for the title as opposed to Britain? Perhaps a declaration elevating the crown of England to that of an Empire? My understanding of the semantics behind a title of 'Emperor' is that it typically involves multiple 'peoples' or 'nations' (of course those are fuzzy designations). In the British Isles, though, there would have been the Scottish, Welsh, English, and Irish people in close proximity under the control of the crown.
After the Great Mutiny in 1857, the British crown seized control of the government of India from the East India Company. The governor was made a Viceroy and the British Empire in India was administered by bureaucrats in London who answered to Parliament. The Royal Titles Act (1876) granted Queen Victoria the title of Empress of India. The act was put forward by the Prime Minister at the time, Benjamin Disraeli, 1st Earl of Beaconsfield (a title conveniently granted to him by Victoria at about the same time).
There were a few advantages to dong so:
Source: Brown, Callum G. and Hamish Fraser. Britain since 1707. London: Pearson Education Limited, 2010. p. 321
The reason why she could not use the title Empress of Britain is because Britain had been firmly established as a constitutional monarchy since the Glorious Revolution and the Bill of Rights in 1688. Parliament governed the United Kingdom as well as the British Empire, and MPs as well as the general public would resent the implication that they were ruled by an Empress, with all its despotic connotations. Unlike the "uncivilized" people of India, the people of the United Kingdom believed that they could govern themselves, and they did. Queen Victoria, much like Queen Elizabeth today, wielded little de facto power and was mainly a figurehead. Disraeli would have been off his rocker if he thought that he could declare Victoria the Empress of Britain and not be run out of Downing Street. Wars had been fought for less in England.