Well, of course it's insecurity, the north feels second-class to the south for a variety of reasons, real or imagined: The south has better weather which automatically makes it more desirable location to be than the cold and wet North. Southern accents are generally favoured over northern accents in some work settings and in most day-to-day settings where they are seen as more refined instead of "t' fockin up norf accent eccy thump!" which by itself causes resentment at the concept that "southerners speak correctly" and "northerners speak incorrectly". Also, northerners think (perhaps incorrectly) that the South is richer than the North, which is a bit of a kick to the North's balls: share the wealth, everything is broken or out of date up here... Given these reasons (and other reasons I cba to formulate into sentences) it's not very inexplicable why people can be insecure/overly proud of being from the North.

Besides, I really can't see any reason why the south could have any resentment towards the north - the south (moreso london) is in most ways better than the north in terms of life quality, hence the phrase "it's grim up north", what could southerners resent about the north? Whereas there are reasons, such as those listed above, for the north to resent the south (again, London specifically).

You're right that there is a London/UK divide, I agree it's not a north/south divide in the traditional sense, but unless you're debating on Question Time, the average guy in the pub won't really bother with specifics, and will just say "bloody southerners", instead of "bloody politically-priviliged and socially-favoured richer parts of london and the south east", even though that's what most northerners probably mean when they talk about the soft/rich south.