There I was getting ready to write about the “Gilbert and Sullivan revival” when the obvious truth dawned that they have never gone away. Even if traditional purveyors like the D’Oyly Carte and Carl Rosa opera companies fell on lean times, a million amateur, university and school performances have gone on as ever, alongside high-profile perennials such Jonathan Miller’s indestructible production of The Mikado at English National Opera, launched in 1986, and the annual “G&S” festival in Harrogate.The pair’s light operas have always excited rapture in degrees ranging from subatomic to unmodified, although for pieces which their detractors hold to be the worst kind of ­anodyne English anti-art, the hatred they excite can be interestingly pass