London is basically one big urban circle. Everything in London is directed towards the centre of the city. Zone 1-6 is roughly a measure your distance from the centre of London, which is also the centre of economic activity with many international HQs, banks, insurance markets, government departments, and startup offices based there. Its also where all of the major entertainment and tourist locations are.

Zone 1 is typically very expensive. Typically, with some variation, it gets less expensive as you go to outer zones. Zone 5/6 is getting to the outer reaches of the city, and historically was much more affordable at the cost of >60 minute commutes into zone 1 and higher fare costs.

You also need to keep in mind that the marginal tax rate can be very high. This is esp true for UK university graduates that typically are repaying what is technically a "student loan", but even the government's statisticians call a tax because almost nobody will pay it off in their lifetime given the repayment terms. Many graduates have marginal tax rates in excess of 50% on incomes above roughly 40k.