While the role of Mr. Putin’s Russia has grown significantly in international diplomatic calculations — from the annexation of Crimea in 2014 to its pivotal role in Syria — Britain’s readiness to mold far-flung events seems to have shrunk back into a twilight of doubt and self-reproach.

Image Alexander V. Litvinenko in November 2006. He died after ingesting green tea laced with radioactive polonium 210.

In July, a long-running inquiry offered a searing indictment of former Prime Minister Tony Blair’s role in the 2003 invasion of Iraq. This week, a parliamentary panel castigated former Prime Minister David Cameron for his intervention in Libya alongside France in 2011. In the current negotiations over Syria, the name of Britain’s new foreign secretary, Boris Johnson, has barely been mentioned.

Mr. Perepilichnyy, 44, died in 2012, three years after he fled Russia, while jogging near his luxurious home on a private estate southwest of London. The death was initially attributed to a heart attack, but traces of gelsemium, a rare toxin used as a poison and derived from a plant grown in the Himalayas, were later found in his stomach.

At the time of his death, he was associated with investigations into a $230 million tax fraud in Russia against an American financier, William F. Browder, a high-profile critic of Mr. Putin.

Mr. Perepilichnyy, who had cooperated with the Swiss authorities and Mr. Browder’s investment company, which were both investigating the fraud, was one of five people linked to the case who have died mysteriously.

Perhaps the most prominent was Sergei L. Magnitsky, a lawyer and auditor working for Mr. Browder, who was arrested and died in prison in 2009 after the Russian authorities denied him medical care.

In the United States, Mr. Browder lobbied for legislation known as the Magnitsky Act that provided for sanctions against Russians accused of wrongdoing at home.