British Prime Minister Rishi Sunak promised 125 anti-aircraft guns and other air-defense technology as he made an unannounced visit Saturday — his first — to Ukraine’s snow-blanketed war-time capital for talks with Ukrainian President Volodymyr Zelenskyy. The air-defense package, which Britain valued at 50 million pounds ($60 million), comes as Russia has been pounding Ukraine’s power grid and other key infrastructure from the air, causing widespread blackouts for millions of Ukrainians as the frigid cold of winter draws near. The package includes radar and other technology to counter Iran-supplied exploding drones that Russia has used against Ukrainian targets. It comes...