Media playback is unsupported on your device Media caption North Korean defector Thae Yong-ho said in 2017 that thinking about the fate of his family in North Korea "breaks his heart"

In August last year, Thae Yong-ho became one of the highest-ranking officials ever to defect from North Korea. In a wide-ranging interview in Seoul, he tells the BBC's Stephen Evans he believes leader Kim Jong-un would be prepared to attack the US with nuclear weapons, but that the regime will one day fall.

There are moments when the usually fluent English of the North Korean defector halts. His voice quivers and he pauses. His eyes grow moist.

These moments of silent emotion come when Thae Yong-ho thinks about his brother back in North Korea.

He told the BBC that he was sure that his family have been punished for his defection. This realisation both grieves him and steels him against the regime.

"I'm sure that my relatives and my brothers and sisters are either sent to remote, closed areas or to prison camps, and that really breaks my heart," he said.

If he could imagine his brother shouting to him in anguish from prison in North Korea, what would he reply?

"That is really a question I don't like to even think about. That is why I am very determined to do everything possible to pull down the regime to save not only my family members but also the whole North Korean people from slavery."

It was his closer family in London who persuaded Thae Yong-ho to defect. He found himself defending the regime to his children, particularly his younger son who was a bright kid in a West London state school.

Image copyright AFP Image caption North Korea's London embassy is in a suburban house in Ealing

The lad grew long hair and wondered how he would be treated back in North Korea. Why, the teenager asked, were North Koreans barred from the internet?

Mr Thae said that in the privacy of their home, they started being honest about the regime, because "you can't lie to your family".

He started leading a double life, addressing far-left groups in Britain about the virtues of socialism while denouncing it at home - and all the time having to tell his sons that they mustn't breathe a word.

He increasingly asked those he met from the West about life in Seoul. North Korean diplomats travel in twos so each keeps an eye on the other. So his questions about the West were often while his (presumably) unsuspecting comrade was taking a break with a visit to the toilet at their favourite West London curry house.

Media playback is unsupported on your device Media caption In December Thae Yong-ho told the BBC he does not regret his decision

Then, eight months ago, Mr Thae and his family vanished from the embassy where they lived, only to turn up in Seoul. He won't say how that journey was made, whether the secret services of Britain or the United States or South Korea were involved.

But he did expand on the process whereby he changed his mind. The son who prompted the change had won a place in Imperial College in London. But he will now study in South Korea - life as an obviously Korean student in London would have been too dangerous, exposing him to the risk of abduction by North Korean agents.

In London, Mr Thae always seemed at ease. He was well-groomed and softly spoken and would have fitted in well at a suburban tennis club - which is precisely what he did.

"I really miss the life, especially in Ealing. Even now I am really sorry for not saying goodbye to the tennis club members because they are really nice and gentle. If possible, I want to say the official goodbye to my old St Columba's Tennis Club members.

"My younger son even joined this club when he was eight. We had a really wonderful coach and he taught the whole family how to play tennis, me my kids and my wife.

"I really miss the English spring and autumn and I really want to say goodbye and thank you."

Media playback is unsupported on your device Media caption This footage appears to show Kim Jong-chol (L) alongside Thae Yong-ho at the Royal Albert Hall in London

As a diplomat, Mr Thae was the servant of a brutal regime and a servant of its leader.

He denies that he ever broke the law. North Korean diplomats are reputed to engage in all kinds of illegality from dispersing fake money to committing fraud, but Mr Thae said he wasn't involved, because in Europe, law enforcement is too effective to get away with it.

The only crime the embassy committed, he said, was to drive without paying the congestion charge and for that it owes £100,000 ($125,000).

He did, however, escort Kim Jong-un's brother, Kim Jong-chul, to an Eric Clapton concert at the Royal Albert Hall.

Mr Thae said the brother was only interested in music. He'd given him a tour of the sites, like Trafalgar Square, and the visitor showed not a flicker of excitement.

Image copyright EPA Image caption North Korea's tests of missiles and nuclear devices continue to alarm and anger the world

Of Kim Jong-un himself, Thae Yong-ho knows little. He said the North Korean leader lives a secretive life. Nobody knows even where he resides.

But he is ruthless, according to the defector, and his ability to wreak harm should not be underestimated. Mr Thae reckons that if his very survival were threatened, he would lash out and destroy whatever he could.

Mr Kim doesn't have the means to attack the United States at the moment but he is developing the ability. The high-ranking defector said that once there was an effective nuclear arsenal, the leader would be prepared to use it.

"Kim Jong-un knows that nuclear weapons are the only guarantee for his rule. And Kim Jong-un, I think, will press the button on these dangerous weapons when he thinks that his rule and his dynasty is threatened."

Image copyright Getty Images Image caption Mr Thae (right) fled to South Korea with his family

Would he even destroy a city like Los Angeles, though the retaliation would surely kill him?

"Yes, because he knows that if he loses the power then it is his last day so he may do anything, even to attack Los Angeles, because once people know that in any way you will be killed, then you will do anything. That is the human being's normal reaction".

Will Kim Jong-un die peacefully in his own bed? "No. I'm sure that Kim Jong-un's regime one day will collapse by a people's uprising."

That uprising, Mr Thae believes will be brought about by the spread of information within North Korea about the world outside.

And will Thae Yong-ho ever see his brother again? "I'm absolutely sure I will and it is my dream to walk back to my home town."