Originally Posted by Jalinth

Partly it is a matter of tradition. London was the centre of the British Empire, with far flung outposts, and acted as a major trading hub. It has also been a very stable country; no revolutions, no invasions, limited corruption, etc... All of which is very popular for headquarters, especially financial ones. Once you've developed into a hub for a particular industry, keeping it up can be a self-perpetuating prophecy. Top stars move to that location to make it, and once they succeed, help attract the next crop of stars. Success breeds success.