What jobs are they doing, the people who buy these houses?
15 years ago, a late-20s single girl-about-town, buys a flat for £150k on her £35k salary as a middle manager. Boy of similar age does the same. Both get 90%+ mortgages, as was easy back then.
10 years ago, they meet and move into one flat and rent the other out. Both flats now work about £250k, so £100k equity each
5 years ago, they sell one flat for £350k and buy a £500k house
This year, they sell the house for £800k and the flat for £500k and can buy a house for £1m, with enough left over to do the loft conversion, new kitchen, and have plenty left over for school fees.
Or they don't sell the second flat and that pays the school fees.
The key is that more than likely got a toe-hold in the market 12+ years, and that first flat will have been somewhere that was a bit grotty at the time, but is now highly desirable - Balham, Shepherds Bush, Hackney, Stokey, Brixton, Clapham. All perfectly affordable when no one wanted to live there