LONDON — The title is “Old Friends” — ideal for a show that feels like one. With Bernadette Peters and Lea Salonga as the marquee attractions, producer Cameron Mackintosh and director Matthew Bourne walk down a boffo, bittersweet memory lane with some of Stephen Sondheim’s most beloved numbers: “Send in the Clowns,” “The Ladies Who Lunch” and “I’m Still Here” among them.