Of course there are jobs in London with a salary of £30k or less. The people doing those jobs tend to live in house shares, often far, far away from central London. Or sometimes they live with family - if you have enough generations here, you might have a family home in zone one without being 'rich'.
Some people live as property guardians, so they have central addresses in cool places, but very little security of tenure as they occupy under a licence rather than a tenancy agreement.
The thing is, for most people, if you have a £30k salary, you have to accept you'll be living with other people or living far away. It's the compromise you make to be in London. It's normal in this city to be living in a house share in your 40s, whereas in most other cities, that would raise an eyebrow. It's just how we do things here.
And of course it's possible to buy in London as a single person without family help. It's just not possible on a salary of £30k.
I know plenty of people who managed to buy in London on a single salary (and I'm not talking in ye olden days either) but the point is, they were high earners, and they accepted they had to live in house shares for many years until they had built up a substantial cash deposit.
The single people who rent whole flats are the ones who can never afford to buy, because their entire salary goes on housing costs. The single people who accept they have to live in a house share for what would be an undignified length of time outside of London are the ones who do end up buying.