LONDON — Britain seized the world’s attention on Saturday by accusing President Vladimir V. Putin of plotting to install a pro-Russian leader in Ukraine, a dramatic late-night announcement that instantly thrust it on to the front lines of the most dangerous security crisis in Europe in decades.

British officials say its release of sensitive intelligence was calculated to foil a potential plot and to send a message to Mr. Putin. They cast it as part of a concerted strategy to be a muscular player in Europe’s showdown with Russia — a role it has played since Winston Churchill warned of an “Iron Curtain” after World War II.

And yet Britain’s moves also bear the imprint of a country eager to set itself apart, two years after it left the European Union. When Secretary of State Antony J. Blinken landed in Kyiv last week for talks about Russian troops massing on Ukraine’s border, his plane taxied past a Royal Air Force C-17 cargo plane that had just finished unloading antitank weapons for the Ukrainian military.

“The U.K. is differentiating itself from Germany and France, and to some extent, even the U.S.,” said Malcolm Chalmers, the deputy director general of the Royal United Services Institute, a think tank in London. “That comes out of Brexit, and the sense that we have to define ourselves as an independent middle power.”

The theatrical timing and cloak-and-dagger nature of the intelligence disclosure, which came in the midst of a roiling political scandal at home, raised a more cynical question: whether some in the British government were simply eager to deflect attention from the problems that threaten to topple Prime Minister Boris Johnson.

Either way, Britain is moving on multiple fronts. It is preparing legislation that would enable it to impose sanctions if Mr. Putin carries out an invasion. It dispatched senior ministers to other NATO countries menaced by Russia. And it has begun engaging directly with Moscow, with reports that its foreign and defense secretaries plan to meet their Russian counterparts in the coming weeks.

Britain’s hard-edge approach was crystallized in a punchy essay by the defense secretary, Ben Wallace. Writing in The Times of London, Mr. Wallace rejected Mr. Putin’s claims of encirclement by NATO and accused the Russian leader of crude “ethnonationalism,” based on what he called the bogus claim that Russians and Ukrainians are one people. The essay made waves in Washington and in European capitals.

“Whether Britain is in the E.U. or out of the E.U., it is always going to push back on Russian bad behavior,” Karen Pierce, the British ambassador to the United States, said in an interview. “Where the Russians are concerned, you’ll always find the U.K. at the forward end of the spectrum.”

But Mr. Wallace is not the leader of Britain’s government — Mr. Johnson is. And the prime minister is caught up in an increasingly desperate campaign to save his job amid a scandal over Downing Street parties that violated coronavirus restrictions. Not only has this political circus crowded out public debate over the British role in Ukraine, but it has also stoked suspicion that Mr. Johnson would welcome a distraction from the flood of pesky questions about garden parties.

Even Saturday’s announcement about a possible coup in Ukraine appeared timed to grab headlines in the Sunday morning papers and airtime on the news shows. Britain rarely declassifies intelligence in this manner, unlike the United States, though it has done so before on issues involving Russia.

“There is no distraction as enticing as war,” wrote Simon Jenkins, a columnist for The Guardian, adding that the only thing more dangerous than a populist leader in trouble was two populists in trouble — in this case, he asserted, Mr. Putin and Mr. Johnson.

Some Conservative lawmakers warn that Britain cannot afford a messy leadership battle at a time like this. Tough talk about Russia also appeals to the Tory right, and critics say some ambitious officials are taking advantage of the tensions.

During a visit to British troops in Estonia in November, the foreign secretary, Liz Truss, posed in military gear atop a tank. Commentators said she looked like she was channeling Margaret Thatcher, which may not be a bad strategy for someone rumored as a potential replacement for Mr. Johnson.

At the same time, there are ample historical and strategic reasons for Britain to take a hard line with Russia. British officials have been furious with the Kremlin since the poisoning of a former Russian intelligence agent and his daughter in Salisbury, England, with a nerve agent in 2018, an operation that Britain blamed on Russia’s military intelligence and that led the British to expel about 150 diplomats.

The Russians have returned Britain’s antipathy, viewing it as the leading edge of American efforts to curb its ambitions and dismissing criticism from British officials as moral posturing, given their country’s imperial past. Britain has done little to stop Russian billionaires from using London as a haven, where they buy up Mayfair real estate and influence in the House of Lords.

While Mr. Johnson has not been as full-throated as his defense secretary, he said on Thursday that “any kind of incursion” by Russia “would be a disaster — not just for Ukraine but for Russia, a disaster for the world.”

The prime minister, preoccupied by his political woes, has largely ceded the stage on Ukraine policy to Mr. Wallace, a British Army veteran who was security minister at the time of the Salisbury attacks. In June, Mr. Wallace deployed a Navy destroyer, H.M.S. Defender, to sail close to the coast of Russian-occupied Crimea in the Black Sea. Russian planes buzzed the ship in protest.

Britain’s action, analysts said, was deliberately aggressive, reflecting a frustration among military officials that its policy has been too reactive to Russia’s serial provocations. These go beyond the Salisbury attack to accusations that Moscow meddles in British elections and has corrupted its politics with dirty money.

Ambassador Pierce pointed out that Britain conducted an independent foreign policy even when it was a member of the European Union. It did, however, take part in E.U.-wide sanctions when it was part of the bloc, something it will no longer do after Brexit. Officials said that was why the government needed to draft new legislation to target Russian individuals and its financial services sector.

Beyond that, analysts said Britain’s determination to be assertive also reflected its post-Brexit identity. Kim Darroch, who was national security adviser under Prime Minister David Cameron, said Britain once refused to supply weapons to Ukraine because it feared they could end up in the wrong hands. Now, those concerns are outweighed by the advantages of acting independently.

“I suspect this is part of showing we’re not bound up with the European Union, which is led by the far more equivocal German view on Russia,” said Mr. Darroch, who later served as ambassador to the United States.

Germany’s equivocation helps explains why the R.A.F. planes carrying the antitank weapons to Ukraine flew a circuitous route across Denmark, avoiding German airspace. A senior British official said that reflected Britain’s close consultations with Denmark and Sweden, and that London did not ask the Germans for permission because it would have delayed a mission that depended on speed.

“The most interesting thing is what it says about how frayed the U.K.-German relationship is,” said Jeremy Shapiro, research director at the European Council on Foreign Relations. “The disunity was on display for everyone who could track the planes.”

Ms. Truss also skipped a meeting in Berlin with Mr. Blinken and her counterparts from Germany and France to discuss Ukraine, sending her deputy. Instead, she traveled to Australia, where she and Mr. Wallace met with officials to discuss a new submarine alliance with Australia, Britain and the United States.

That seemed an odd choice in the midst of a mushrooming European crisis. But it underscored Britain’s commitment to Asia, another cornerstone of Britain’s post-Brexit foreign policy. It also, analysts said, helped Britain avoid the perception of being unduly subservient to the United States.

“They have to work carefully not to be seen as a poodle,” Mr. Shapiro said. “They want to show that they are an extra-regional player.”

Michael Schwirtz and Michael Crowley contributed reporting from Kyiv, Ukraine, and Anton Troianovski from Moscow.