Well it depends on how you define power?At its height the British Empire ruled over 1/3rd of the world's population and 1/4th of the world's land area. Its navy was larger than the next two combined and while it maintained a small army in peacetime, in wartime it could raise an army over 8 million strong. In world war 2 the British empire was able to stand against the German and Italians practically alone for nearly a year. The British Empire was able to outlast the French, German, Russian, and Austrian empires, before finally collapsing. So clearly the British Empire was definitely worthy of the title "Super Power" .

Source: The Rise and Fall of the British Empire by: Lawrence James is a good read if you want to get an idea of the scope of the British empire.