Dramatic news broke this week that Thae Yong Ho, number two at the North Korean Embassy in London, has defected with his family to South Korea.

He is the most senior diplomat to defect in years, possibly the most important since Hwang Jang Yop, one of Pyongyang's chief ideologists, fled in 1997. Thae Yong Ho's arrival in South Korea has been instantly heralded as a triumph by the Park Geun-hye administration. His debriefing promises a potential treasure trove for South Korea's National Intelligence Service. In the course of his career, specialising in the UK and Europe, Thae is likely to have accumulated substantial knowledge about Pyongyang's overseas finances – licit and illicit. His connections and influence ran deeper than a deputy position at the embassy suggests.

Over several years as a North Korea specialist in the Foreign and Commonwealth Office I came to know Thae Yong Ho about as well as a Western diplomat could hope to. While our interactions were professional, occasionally tense, never wholly trusting, we developed a rapport nonetheless. As this BBC profile makes clear, Thae appeared strangely at home in suburban North London. Like some other North Korean diplomats, but to a greater degree than most, he had a disconcerting ability to compartmentalise the varied requirements  of his portfolio.

There is a quick-witted, calculated confidence to his conversation, and I think he enjoyed intellectual sparring as much as he enjoyed tennis, at his local club in Acton. In the course of his two postings to London, it appears that enough of the Anglo influence rubbed off in ways that led him to profoundly question his allegiances, and to plot his escape for months – if not years – in advance.

Initial reports said Thae was bound for a 'third country'. This may have been misinformation to disguise the fact that he was still in transit for Seoul. Alternatively, it could be a pointer that South Korea was not his first-choice destination. Although defectors from the North are automatically entitled to South Korean citizenship, some have chosen to live elsewhere. However, such a high-value catch would have triggered an all-out effort on Seoul's part to bring him to South Korea. Whether he will remain there with his family remains open to question. If so, they will face restrictions on their movements and require around-the-clock protection against assassination or kidnap.

While a single defection should not be over-read as an indicator of instability, this will badly rattle the North Korean regime. Statements subsequently attributed to North Korea's Atomic Energy Institute, claiming that Pyongyang has resumed plutonium reprocessing, look suspiciously like an effort to bury bad news.

Increased counter-intelligence measures for North Korea's overseas officials must surely follow. Diplomats will find themselves at least temporarily reined in and under intense scrutiny, for fear of copy-cat defections. Family members may be recalled to Pyongyang. There could be direct repercussions for Foreign Minister Ri Yong Ho. Thae served his first posting in London under Ri, before the latter was appointed as the North's chief nuclear negotiator and then foreign minister. Ri will face tough questions as to how his protégé's ultimate act of disloyalty went undetected. Thae had a very high level of trust vested in him, including escort duty when Kim Jong Un's brother flew in to attend Eric Clapton concerts in London in 2015. Thae was privileged to be accompanied by his children on a foreign posting. With his escape a fait accompli, someone will have to face punishment, potentially including extended members of his family.

In my experience, no matter how much Thae affected the tweedy accoutrements of British gentlemanly living, there was always a roguish edge showing through, in a grin or glint of the eye. Regardless of his feelings towards Kim Jong Un's regime, I believe he will remain proudly North Korean at heart. If he lives long enough to see re-unification he may even have a returning role to play there. Before then, he will need every ounce of cunning to navigate the next phase of his career.