I think that's overstating it. My first job in London I was on £28k and I was ok but wouldn't want to go lower. Take-home pay was about £1800/month, I spent £850/month rent (Zone 3, noisy loft conversion in an old building), maybe £450/month general shopping (food and clothes and the like), £200 going out and the rest on occasional stuff like a computer or holidays.

You can live much cheaper - I've seen 11k claimed by someone who shared a tiny basement and cooked all their own meals off the cheapest supermarket food - but you're making significant lifestyle compromises.