I think the key thing is history. The UK developed the nuclear bomb with no help from the USA. As a consequence the UK was not part of the Marshall Plan and as well as enduring a 'rebuild from nothing' there was also the phenomenal cost of Windscale and all of the rest of the infrastructure, a cost borne without the industrial base the USA had - ruins vs. vast and intact. The Manhattan Project was a similar cost to the moon landings. Sadly the UK invested in their own version of the former rather than the latter. But nobody else had the bomb apart from the USA and the USSR so having the bomb was the UK's ticket to being a world power rather than entirely under the sphere of influence of one superpower or the other, directly or by proxy.
The Kim family stepped in for Japanese Emperor as far as being divine father figure for the nation, at one level the 'world power' rationale is the same as what the UK had to play post WW2. Much like it has been in other parts of the world the enemy is the USA and it is quite clear that the goal of the North Korean programme is to be able to get some missile with some nuclear weapon at least as far as a big city in the continental USA.
I am no defender of the crazy kid, however, there is some serious stuff behind the attitude of North Korea. They do not want their culture to be Americanized, they do not want advertising and they do not want to be yet another consumer society. A whole smorgasbord of Western beliefs are just not wanted by them. There is also the matter of unification. Their idea is to have a federation, one country with two states, governed by their respective leaders and with the one seat for the federation at the UN. That is what they want and without any outside interference. So those nuclear weapons are pointing outside of the Korean peninsula and that would be the situation of a Korea united on the North's terms - 'without outside interference'. A unified Korea with weapons of the nuclear flavour would keep all the neighbours at bay on the MAD basis so a unified, federated Korea (which sounds reasonable to some in the South) is just not going to happen.
Seoul is not going to get annihilated by the North. Ignore the Kims for a moment and look at what the Koreans could achieve. A united Korea would be second on the stage after the USA in terms of GDP+population+trade+influence and if you add the nukes then they are totally deserving of taking the UK seat or anyone else's except the Americans in the security council. That might sound far fetched, but I believe that this is an obvious prize in Korea and destroying Seoul goes against that.