THE billionaire who came in from the blue. Unheard of in London a year ago, this chattering city now finds him settled at the top of The Sunday Times millionaire list—the sizing up of big money stardom that has turned business success into a society coup. And in a city where such money rankings sit like epaulets on a general's shoulders, Laxmi Mittal, as the seventh richest person in Britain, is now Billionaire-in-Chief among Asians.