LONDON—Britain on Friday slapped sanctions on seven men it said were Russian intelligence operatives responsible for the poisoning of Kremlin critic Alexei Navalny. It also issued a joint statement together with the United States warning Russia over chemical weapons. It did not outline any specific new actions taken by Washington but again condemned what it described as an assassination attempt against Navalny. Navalny was flown to Germany for medical treatment after being poisoned in Siberia on Aug. 20 last year with what Western experts concluded was the military nerve agent Novichok. Moscow has rejected their findings and accused the West of a smear campaign against it. An updated version of the British sanctions list published by the government on the first anniversary of Navalny’s poisoning included seven new names. “The sanctioned individuals are directly responsible for planning or carrying out the attack on Mr. Navalny,” a Foreign Office statement said. …