The location of the 0 longitude line has less to do with the British colonial empire as such, and more to do with the fact that the British were the first people on Earth to invent a way of measuring longitude at sea accurately (John Harrison's famous marine chronometer). Of course these are ultimately related facts - the same package of technology that allowed the British to produce people like John Harrison who were capable of innovating a marine chronometer also allowed them to produce ocean-going ships that could conquer a global maritime colonial empire (and that had a use for marine chronometers).

There was in fact a rival 0 longitude line passing through Paris, France, that the French used for many years before standardizing on the British system; although this system would equally have resulted in the 0,0 point being somewhere in the ocean west of the African coast.