Let's ignore people with family already in London, or those with the bank of mum and dad.
How can two young professionals even begin to think of buying a house in a decent area of London?
Just looked at fairly boring but nice 3bed terrace houses in Fulham are looking for £1.5mil.
Think of two lawyers or bankers, on £100k a year, or more. They'd still be nearing 35+ when they could consider buying.
That's not even thinking about "normal" people! Even grotty flats are £500k+