I agree that London house prices (and rentals) are, IMO, unjustifiably expensive. A lot of people are coming to the city for work - quite reasonable - but when you have, eg, 3 young professionals in a flat share they will inevitably have greater purchasing power than a family with 1 or 2 earners. This disparity increases if those young professionals are earning above average salaries. They may be willing, like other people have mentioned, to spend a few years here then move out and they accept the financial hit of paying £700-900pcm (or more!) for one room for the time being.
Some people are hugely underestimating the relevance of community. A lot or people, across the UK and the world, don’t move away or far from where they grew up - or they do and then move back. Sure, plenty of people do move far away, they even emigrate, but a lot of people also don’t.
I grew up in London. I still live here because... where else am I going to go?! I have no real reason to leave because at least there is a lot of work available here for me, plus I have all of my family in this city. I could move to a few cities in the north in terms of reasonable job prospects for my profession, and I’d definitely be able to afford to rent something bigger and nicer. But then I’d be in a place I have zero connection to, hours away from family, and career progression would be more limited than in London. It just doesn’t make sense to me.
I grew up on a housing estate and live on one now, and I’m not sure where some posters think people with limited finance will have the resources let alone the rationale to move to a different city. It might not be great here, but it is what it is, and at least people have their friends, their family, and they know the place. Why exactly would someone renting a small flat, working in a low paid job, who has lived here all their life, with all of their experience and connections here, say, “Hey, do you know what, I reckon Manchester has got to be better than this”. And genuinely, is it? Yes the rent is ridiculous, and is subsidised by Housing Benefit/UC really. But is being poor or low paid in Manchester better than being poor or low paid in London? I have my doubts. So people just keep on keeping on.