I was thinking about this in the queue to see the Queen. (I never actually met her whilst she was alive, so somehow joining the great unwashed to pay my respects to her in death became just that little bit more important.)
One answer must of course be "fashion", though there's a sense in which the English follow a sort of anti-fashion. Apart from maybe haircuts and brand labels, it will be interesting to see how the screen dramatists of the future try to evoke the style (or lack of it) of today in "period" settings.
Because the truth is that there's something altogether timeless about the ahistoricity of today's lack of fashion. The same old mixture of shabbiness and garishness, whose only constant feature is its failure to please the eye, could be of any decade from the 1990s to the 2020s - an entire generation effectively devoid of elegance or style.
One is tempted then to blame "consumerism" - at least in the sense of Edward Bernays and all his brain-washing ad campaigns. Self-expression trumps good taste, and so the only thing they all have in common is that they dress badly. "You can't tell me what to wear. I'll wear what I like." And once the very idea that there is such as thing as dressing nicely has disappeared, all that's left is dressing horribly. (One can't help ponder that this is the sort of explanation that would appeal to the likes of Adam Curtis - and even the King himself.)
But above and beyond there's something more to it than that. The English now have precious little sense of dressing for an occasion. They no longer dress appropriately for town or country, for weekdays or Sundays, for work, for leisure, or for pleasure, let alone for times of national mourning. In fact of course Sundays and workdays have changed places in people's priorities, since everyone started working in offices rather than in real jobs (and stopped going to church on the sabbath). One dreads to think what the new culture of "working from home" will produce. Will the next royal lying in state see people queuing up in their Jim Jamz?
So if the new King really is set on restoring "harmony" - to the planet, to the realm, to the lives of his people! - encouraging men to dress nicely would been an obvious place to start.
UPDATE: Paul Joseph Watson adds his comments here. "People dress badly because they don't like themselves." Well, there's certainly something to that.
