Keith Lockhart is used to conducting on a big stage, what with all of America tuning in to watch him lead the Boston Pops on the Esplanade every Fourth of July. But on Monday, the world will be watching as Lockhart conducts the BBC Concert Orchestra in the Diamond Jubilee Concert for Queen Elizabeth II. (We looked it up: The Diamond Jubilee celebrates the Queen’s 60th anniversary on the throne.) The show, which will take place on an enormous stage set up in front of London’s Buckingham Palace, will feature a calvacade of stars, including Paul McCartney, Elton John, Lang Lang, Stevie Wonder, Renee Fleming, Andrew Lloyd Webber, Shirley Bassey, and Annie Lennox, among others. Friday, we reached Lockhart as he was getting ready to go. “It’s the kind of thing you don’t say no to,’’ he said, laughing. Although Lockhart has been the principal conductor of the BBC Concert Orchestra for two years, and has known about the concert for some time, he didn’t think he’d be available due to the Pops’s schedule. “There was a little bit of a last-minute conductor issue, and I decided I could actually do it and get back for the next Pops concert,’’ he said, referring to Tuesday’s show. “It’s a tight turnaround, but if you’re fortunate enough to be asked, it’s good.’’ Wondering what the orchestra will be playing? Lockhart is, too. “I’ve only known I was doing this for about 24 hours so I’m not sure of the full details. I know we’re performing with Renee Fleming and Lang Lang and there will be some Puccini, Gershwin, and “God Save the Queen.’’ Finally, we wondered how a Yank landed the gig leading the BBC Concert Orchestra. “The war’s been over for a long time,’’ said Lockhart.