Alternatively, why was it called the British Empire when it was really the Kingdom of Great Britain? To me, having a empire without an emperor is as nonsensical as having a kingdom without a king. They seem to imply each other, yet it didn't work that way. Is there a good explanation for it?
closed as off topic by Jasper Loy, simchona♦, FumbleFingers, Shyam, kiamlaluno Jul 4 '12 at 9:16
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The reason is simple the unpredictable course of history. It was first proposed to George III that he take the title of Emperor upon the union of Great Britain and Ireland in 1801. However, he rejected this title.
Then in 1876 Victoria assumed the title of Empress of India, which had previously been the title used by the Mughal Emperor (as the Padishah). This title held until the 1947 independence of India and Pakistan. George VI and Elizabeth (later Queen Mother) were the last Emperor and Empress to hold that title.
Under the Commonwealth, the member states are assumed to be equal and so there is no Emperor. Elizabeth is legally the Head of the Commonwealth.
The ruler of the British Empire was indeed an Empress (at least until 1997, when Hong Kong was given back to China). She also happens to be a Queen.