I'm from the north, live in London, and go back to the north a lot.
Obviously this: There are all sorts of people of course.
And particularly in London, which is huge, the areas are all different anyway. I'd say it's noticeable that people are richer/swisher around expensive parts of West London, like the Kings Road will have more 'well turned out' people than a northern shopping mall, but also more than a Croydon shopping mall or similar. That's about money and tribes. I don't especially think that sloaney look is more attractive or interesting though, often the opposite. It can feel a bit uninspired.
What I find is that in cities in the north-west (can't speak for the rest), there will be some extremely well turned out people - women who get their hair, nails, make-up done and dress up, and the male equivalent. Whether you find that look good-looking is again a matter of taste. But what I also find - and like - is that people generally give less of a shit if you're not so dressed up, so I like that I can relax and go out in scruffier/more comfortable gear and not get judged. You can be as you choose and people take you as they find you, it's not so much about appearances, though of course there are still some specific tribes.
So I definitely don't think it's about Londoners being more comfortable in their own skin or anything like that. I like both places though and the mix of people makes both interesting and different. There's not really a 'better' about it.