President Trump mixed high-level diplomacy with schoolyard taunts as he kicked off his overseas visit meeting with Queen Elizabeth II at Buckingham Palace and later touring Westminster Abbey -- while sparring on the side with one of his biggest British critics, London Mayor Sadiq Khan. The London itinerary is part of a packed overseas agenda that includes a state dinner at the palace and D-Day commemoration ceremonies. At their initial visit to the palace early Monday morning, the president and first lady Melania Trump had a private lunch with the Queen, later inspecting a collection of artifacts including an 18th-century...