Most staff working in central London live outside London well beyond zone 6 as these areas inside the worlds best transport system are well out of their reach. How does someone get to Basingstoke, Ayslesbury, Southend via public transport in the early hours of the morning? It's not achievable to live anywhere within zone 6 on £40k.
I'd be interested to see how people who work in central London and clock off at 2am live outside of the city itself. Very few I'd imagine, it's a completely ridiculous situation and one where you'd consider moving /changing job.
You can live in zone one on 40k anyway, though obviously can't buy.