The metropole (from the Greek metropolis for "mother city") is the British metropolitan centre of the British Empire; i.e., the United Kingdom itself. It is sometimes used even more specifically to refer to London as the metropole of the British Empire, insofar as its politicians and businessmen determined the economic, diplomatic, and military character of the rest of the Empire. By contrast, the periphery was the rest of the Empire, outside the United Kingdom itself.
Metropole and periphery
The historiography of British metropole-periphery relations has traditionally been defined in terms of their distinct separation, with a pronouncedly one-way, near-dictatorial channel of command, communication, and control proceeding outward from the center; the metropole informed the periphery, but the periphery did not directly inform the metropole. Hence, the British Empire was constituted by the formal control of territories, by direct rule of foreign lands, instigated by the metropole.
More recent work, starting with that of John Gallagher and Ronald Robinson in the 1950s, has questioned the traditional definition, positing instead that the two were mutually constituitive and maintaining that, despite the apparent temporal inconsistencies inherent in their separate existences, each formed simultaneously in relation to the other. Gallagher and Robinson were socialists, observing the rise of the economic power of the United States in the developing world at a time when the African colonies of the British Empire were being granted independence; both scholars held that British and American "empires" were ultimately developed along similar lines.
In the context of Gallagher, Robinson, and Adlai Stevenson's theories of "free trade imperialism", the use of soft power, primarily through the employment of British capital, allowed the United Kingdom to extract concessions, namely free trade for British manufactured goods, just as readily as if they had engaged in a costly military occupation of the territories. In this interpretation, the economic informal Empire of the periphery constituted a formal "Empire" just as surely as the metropole did.