A Ukrainian deliveryman in the English town of Basildon watched the first days of the Russian invasion of his country on television before deciding he had seen enough.

On Friday the deliveryman, Oleksandr Bilyy, packed his car, said goodbye to his wife and headed to his homeland, hoping to join the fight.

“Nothing will change if I stay in my house,” Mr. Bilyy, 39, said in a telephone interview as he raced toward the English Channel. “We’re ready to kill some Russians. Every last Russian on our land.”

Mr. Bilyy is forming part of an early vanguard of Ukrainian citizens in Britain who are answering a call by President Volodymyr Zelensky to help defend Ukraine.

Britain’s foreign secretary, Liz Truss, said Sunday that the government would not oppose those who want to take up arms for the Ukrainian resistance. “If people want to support that struggle I would support them in doing that,” she told the BBC.

Britain is home to about 35,000 ethnic Ukrainians. Community leaders in London said Sunday that they knew of at least 40 people who had made plans to head home and that many more would probably follow.

Mr. Bilyy has taken up arms against a Russian threat before. In 2014, while still living in the western Ukrainian city of Lviv, he joined the army after Russian-backed separatists seized part of the country’s east. Mr. Bilyy has an added motivation this time: a 21-year-old son in the Ukrainian Army who is getting ready to deploy.

“I need to try my best to be there in time to protect him,” Mr. Bilyy said. “I don’t have time to wait for anyone.”

Volunteers with medical expertise have also been spurred to action.

Dr. Dennis Ougrin, a children’s psychiatrist in London and a Ukrainian native, spent most of Friday with his wife, Roxanne Litynska, gathering donated medical supplies around London. At the Ukrainian Social Club, in the city center, the pair loaded up their Volvo with bandages, syringes and field dressings. With help from a Ukrainian charity, they also bought a portable ultrasound machine, which can be used to find shrapnel embedded in people’s bodies.

“I’m not really good with guns,” Dr. Ougrin said. “I work better with children.”

Word of his effort spread quickly. By Friday, people were approaching him in the street and at restaurants, pressing cash into his hand. A local dentist came forward toting a large bag of supplies. “It’s amazing; I don’t even know many of these people.” Dr. Ougrin said.

The plan, he said, was to drive to Poland’s border with Ukraine, a 20-hour trip, stopping along the way to pick up supplies gathered by other Ukrainians. At the Ukrainian border, he would pass the items to other volunteers to distribute at hospitals and clinics.

In 2014, Dr. Ougrin and his wife took three British vehicles to the Ukrainian Army at the frontline. With their driver seats on the right-hand side, “they help to fake out the Russian snipers,” Ms. Litynska said.

One van, a Mercedes-Benz Sprinter, was converted into a portable freezer to retrieve the bodies of soldiers killed behind enemy lines. “After eight years it’s still operational,” Ms. Litynska said.

“Obviously, we would hope this is the only trip we would need to make,” Dr. Ougrin said. “But it all depends on how the war goes ­— we could be at this for a very long time.”

In the basement of the Ukrainian Social Club, a group of men, most of them Ukrainian Army veterans, discussed the latest attacks with a sense of urgency.

“They’ll fight until the last one is standing,” said Roman Azarov, a former military officer. Since 2015, he said, he had been engaged in military training with about 20 Ukrainian expatriates at a camp outside of London. Many at the club were planning to leave for the front lines as early as Sunday.

The Russian invasion appears to be catalyzing many on Europe’s far right as well. Internet sites frequented by right-wing extremists have been abuzz with talk of raising money and recruiting fighters.

Since the first Russian military intervention in 2014, Ukraine has emerged as a hub of white supremacy. “Eight years of conflict in eastern Ukraine, coupled with active conflict across Ukraine today, presents a concerning theater of operations for the far right on Europe’s doorstep,” Ali Soufan, a security expert in New York, said in an email.

The instability “offers white supremacy extremists the same training opportunities that instability in Afghanistan, Iraq, and Syria has offered jihadist militants for years,” he said.

Last week OC, a French white nationalist website, published a pro-Ukraine statement on Telegram, an encrypted messaging app, that encouraged subscribers to donate to Carpathian Sich, a Ukrainian paramilitary group. The post was located and translated by the SITE Intelligence Group, which tracks extremist organizations. A subsequent OC post said, “Just like the U.S.S.R., Putin will be defeated,” by aligning “French nationalists” with the Ukrainian people.

On Sunday evening, after driving for two days, Mr. Bilyy, the would-be fighter, arrived at the Polish-Ukrainian border town of Korczow to discover he had many Ukrainian friends also heading toward the war. Five men from the United States, the Czech Republic and Poland were there, eager for a ride into Ukraine.

“The whole world is here,” Mr. Bilyy said. “We are all ready to fight. We will stay there until the end.”

With everyone in the car, they drove at dusk across the snowy border toward Lviv.