The London audiences aren't wrong. "The Phantom Of The Opera" is romantic musical theater hokum in the grand manner - hokum cordon blue - and it justifies the feverish buildup that has given it a $16,500,000 advance. It's good for a Broadway run of several years. Andrew Lloyd Webber has taken the Gaston Leroux potboiler about the love-crazed disfigured genius who lives in the catacombs of the Paris Opera and fashioned it into a thrilling and musically rich mass legit entertainment. The 19th century period spectacle, scenic legerdemain, soaring melodies and exceptional singing are at the service of an involving and piquantly offbeat love story, all of it staged with brilliantly organized flair by Harold Prince, back in top form.