Merrily We Roll Along,” “Sweeney Todd” and final show “Here We Are” are all playing at the same time. Over in London is a sublime revue of favorite Sondheim standards called “Old Friends” at the Gielgud Theatre that’s led by a deep-feeling Bernadette Peters and Lea Salonga.Two hours and 30 minutes with one intermission. At the Gielgud Theatre in London.