Looking Back on England's Powerby ANNE WILSON; Anne Wilson, who lives in London, is the author of ''Mixed Race Children,'' which was published in this country this month by Allen & Unwin.July 26, 1987LEAD: Slowly, the northern cities of England are being transformed. The mills and factories, which, in the 19th century, clattered and smoked to make Britain the world's foremost industrial power, are being saved from decay to find new grandeur as industrial museums and monuments to the Industrial Revolution.(read article)