In June, NBC News visited a London suburb that has become home to hundreds who have fled North Korea, which on Sunday claimed to have successfully tested a hydrogen bomb designed for an intercontinental ballistic missile.

LONDON — The defectors who now call "Little Pyongyang" home paid a heavy price to get there.

"My big brother is currently in prison in North Korea and he's not getting released any time soon," Kim Kwang Myong told NBC News. "The reason he is in prison is because of us fleeing the country."

"It was a life-or-death decision"

Around 5,300 miles from North Korea's brutal dictatorship, the bland commuter suburb of New Malden has become an improbable home to hundreds of escapees.

With its sleepy town center and large, semi-detached houses, it is a world away from the human-rights abuses, poverty and isolation of their homeland.

Kim's story is a common one. Like many people here, he fled North Korea but left family members behind. The regime often exercises a merciless policy of collective punishment against remaining relatives, sending them to labor camps, or worse.

Kim, a 46-year-old carpenter, knew this. But he said inhumane conditions in the country gave him no choice.

"When I left North Korea it was a life-or-death decision," he said, speaking while colleagues around him noisily renovated a new Korean butchery in New Malden.

Kim Kwang Myong Carolina Reid / NBC News

Picking his moment, Kim bribed some border guards to vacate a stretch of North Korea's river border with China, allowing him to cross with his wife and two children. That was 20 years ago.

He has since lived illegally in China and then in South Korea before coming to the U.K. four years ago.

Kim has tried to help his relatives who stayed behind by sending them money. It was this that led to them being punished after authorities discovered the transactions.

"My younger brother was sent to prison and stayed there for one year but got released. But my big brother has not been so lucky," he said.

His family has suffered, and that knowledge weighs heavily on Kim.

"We are talking about two different worlds, really," he said of the difference between his new and old homes. "The reason why we fled from there is because life over there is really hard. It is simply impossible to live."

Nazi-style camps

Totalitarian North Korea restricts every aspect of public life, throwing people into Nazi-style camps for crimes as petty as "gossiping" about the state. Ordinary citizens are not allowed to access the internet or the international press, instead having to rely on the propaganda of North Korea's state-run media.

The pariah state has come into sharp focus internationally as North Korea's escalating missile and nuclear tests have collided with President Donald Trump's uncompromising rhetoric.

Trump has refused to rule out military action — an option experts warn would likely result in deadly retaliation against American allies and a war that could kill up to 1 million people.

Nonetheless, Kim Jong Un's regime continues to promise to build a nuclear weapon capable of hitting the U.S. mainland.

So North Koreans wanting to flee is understandable. But their choice of a new life in a nondescript suburb on the other side of the planet needs more explaining.

New Malden sits on the edge of the British capital and the rural county of Surrey. It's the type of place that, despite its ZIP code, most city-dwellers might dismiss as not-really-London.

"We were at a crossroads whether to be sent to prison or fleeing from the country"

It boasts a huge Korean population, officially around 3,500 but with some estimates putting it closer to 20,000 in the wider borough.

Most of these are South Koreans who flocked here, so the theory goes, because it used to be the site of Britain's old Samsung headquarters and the residence of the South Korean ambassador.

Of this community, several hundred are North Korean — making it the largest such community in Europe and one of the biggest outside the Korean Peninsula.

North Korea sees the United States, South Korea and Japan as its arch enemies, so many defectors feel they cannot resettle there. This is either because of lingering misgivings about countries they've been taught to hate, or because they fear their left-behind family members will be treated more harshly if they go there.

The New Malden area of London. Carolina Reid / NBC News

There were 655 North Koreans registered as living in the U.K. as of 2015, according to United Nations figures. However, the European Alliance for Human Rights in North Korea believes the actual is likely closer to 1,000. Most have settled in New Malden.

Its residential side-streets are quaint and leafy, giving a hint of the English countryside that's just five miles down the A3 highway.

But look closer and hints of Korea begin to make themselves apparent. Characters of Hangul script are daubed on an upstairs window above a shoe-repair shop. There is a Korean travel agent and a handful of Korean stores and restaurants.

One of these, a Korean barbecue, is owned by a 34-year-old restaurateur who only wants to be identified by his nickname, King, amid fears of his family being punished.

His story is also one of heart-wrenching do-or-die decisions and a family ripped in two.

King, 34, is among the North Koreans living in New Malden. Carolina Reid / NBC News

King fled his homeland with his mother and sister when he was aged 18. But his father, a high school teacher who felt loyalty to his job and fear of the regime, stayed behind.

"He didn't agree with us leaving," King told NBC News while grilling meat over a high flame in his restaurant's kitchen. "He knew what would happen to him but he also knew there was nothing for me in North Korea."

As punishment for his family's actions, his father was fired and sent to a labor camp.

Nowadays, King only gets to speak to his dad once every two or three years, on the rare occasion his father can get an illicit cellphone capable of making international calls. He hasn't spoken to any of his friends since he left.

"Yes, I miss them, of course, but I have friends here now," King said. He quietly added: "It's difficult to talk about my life here and my life in North Korea."

In a way, his family's hand was forced. Before they fled, his aunt had already escaped to China and they risked being punished by proxy if they stayed put.

"We were at a crossroads whether to be sent to prison or fleeing from the country," he said.

'The difference is like hell and heaven'

Although the town has a distinct Korean tinge, the cultural hub is at an industrial park around 15-minute walk from its center.

Here, the Korea Foods superstore acts like a wormhole, whisking shoppers away from the humdrum suburbs and into the heart of downtown Seoul.

A cashier at the Korea Foods superstore. Carolina Reid / NBC News

Even compared with metropolitan London, the vibrant array of colorful packaging, mysterious vegetables, and staff who speak little-to-no English are an arresting cultural vignette.

In the rooms above the superstore are the unglamorous offices of Free NK, a North Korean newspaper run by Kim Joo Il, another defector.

"If you actually compare two lives, one in North Korea and the other one in New Malden, the difference is like hell and heaven," the 43-year-old told NBC News.

When he lived in North Korea, he served as an officer in the Korean People's Army and it was his job to catch defectors. He knew the risks of trying to flee.

"They were all dealt with by military law, which meant public execution," Kim Joo Il said.

According to him, the country's feared secret police has a network of spies so extensive that one out of every three citizens is an informant.

"Your lives are under surveillance every single moment," he said. "Kim Jong Un has told his people that the tiniest thing, even the drop of a needle to the floor, should be reported back to him."

An array of colorful packaging at the Korea Foods superstore in New Malden. Carolina Reid / NBC News

Despite being aware of the potential consequences, he decided to take his chances and make a break for it across the Chinese border.

He had already been to China in 1999 on official business. So using that trip as a reconnaissance mission, he returned in 2005 and in the dead of night swam across the River Tumen, which separates the two countries.

"This is not a choice that you make in a day," he said. "This is based on a long-term emotional process. You make up your mind to escape from North Korea, and then you give up on the idea, and then you make up your mind again, and then you give up again. You go through this process so many times you cannot imagine how many times."

During this time he said that his "emotional ups and downs were indescribable."

Kim Joo Il was single when he fled, but he had to consider the consequences his escape would have on his remaining family members.

"It's not just the family that you have in mind, you've got to actually be prepared to die, really, while escaping," he said. "Personally it took me eight years to finally make up my mind and in the eighth year I made my escape."

From China, he walked, hitched rides, and scraped together enough money for the occasional train or bus fare. He traveled through Vietnam, Cambodia and finally Thailand, where he got a plane ticket to the U.K.

He publishes the Free NK newspaper both in print and online, employing around five members of staff — both North and South Koreans — and highlighting the atrocities the regime is inflicting on his countrymen.

Kim Joo Il, who runs the Free NK newspaper, recounts his escape from North Korea. Carolina Reid / NBC News

Not only does he circulate the newspaper locally, he sends the digital files to South Korea where they are printed out, attached to balloons and dropped over North Korea as anti-regime propaganda.

"We've got to have the right direction of the wind," Kim Joo Il said, proudly showing off an example of one of these airborne packages, sealed in a clear plastic bag. "The wind is really crucial to sending the newspapers to North Korea."

Now well-known as a figurehead in the New Malden community, Kim Joo Il is determined to be a thorn in the side of the dictatorship.

Just 10 miles to the north, in the London suburb of Ealing, is the North Korean Embassy to the U.K. Given the regime's apparent willingness to assassinate dissidents abroad — Kim Jong Un's own half-brother, for example — does he worry about his own safety?

"That goes without saying," Kim Joo Il said. "Threats to our own safety are always lurking in the background."