The title basically, why did Queen Victoria and not Empress Victoria reign over the British Empire? Or, going back even further to before George III gave up most of his power, why was it King George III and not Emperor George III ruling the British Empire?
Queen Victoria was her domestic title, however she was technically an Empress after 1876. Why?
The short version is that it hadn't really mattered up until then, but then in 1871 Germany unified and Wilhelm I became Emperor of Germany. This led to a pressing problem for British diplomats. In the 1870s Great Britain was undoubtedly the premier power in Europe, and it was ruled merely by a Queen. The national dignity in this time of intense European colonial competition was imperative. Furthermore, in India Victoria was technically overlord of a collection of over 500 "princely states" in India. These states ranged in size and importance, however technically their rulers were mostly Maharajahs, a kingly title. The British referred to them as princely states, but they were in essence leftover kingdoms the British had suzerainty over. Some of them had previously been ruled the Mughal Emperor, who had been defeated in the 19th century, so the need for a replacement overlord title was there.
Benjamin Disraeli, the conservative prime minister, passed the Royal Titles Act 1876. It was suggested Disraeli was in pursuit of ennoblement himself, as a former Prime Minister, later in the 19th century, he was raised to an Earl in thanks by Victoria.
British coins between about 1880 and 1948 contained the script Ind.Imp, short and Latin for "Emperor/Empress of India".