“Better be good,” Gregory Porter quips, just before opening his mouth to sing for a crowd of thousands in one of London’s most historic venues. And it is good. One Night Only, a program venerating Nat “King” Cole (along the lines of Porter’s 2017 studio album of his songs), finds the authoritative baritone in a lush setting with a 70-piece orchestra led by famed arranger Vince Mendoza. But despite a prevalent focus on ballads—including the murderous, Cole Porter-penned “Miss Otis Regrets,” subject to intriguingly knotty reharmonization—Porter preserves the core of swing and soul that’s so central to his small-combo identity. Chip Crawford’s piano solo on “Pick Yourself Up,” Tivon Pennicott’s bebop tenor sax turns on “L-O-V-E” and “Sweet Lorraine,” bassist Jahmal Nichols’ bluesy aside on “When Love Was King,” the creative funk abandon of the quintet on the finale encore, “Don’t Lose Your Steam”: This is Porter in his element, formal setting or otherwise.