And yet, I can do a bootstrap startup and my girl is unemployed (just finished her PhD and now looking for an academic position) and we do alright on a combined salary around ~£1.1k per month take home.
Then rent in London: Shared house/apt = ~£650 per month, private apt = ~£1.2k per month. Deals are to be had if you can wait for them and do the legwork... those are the averages my friends pay.
Now... on a £40k salary, the PAYE tax calculator here: http://www.listentotaxman.com/ shows that you'll be taking home £2.5k per month.
I lived in South Wimbledon (the not so nice part, but still better than surrounding areas - i.e. you won't get stabbed) paying £400 for a double bed in a shared house. It is zone 3, so 30 mins to get to zone 1 on the tube, but I liked it.
And yet, the post you are replying to shows 2 people living on ~£1.1k Net monthly income in London, and already shows rent isn't as extravagant as people think.
You can support someone else, on a single £40k wage, in London, in an 1-bed place (assumes you're supporting a partner).
Maybe you can't afford a car, or to eat out several times a week, or to take lots of holidays... but £40k goes pretty far and providing money isn't being spent frivolously it easily supports 2 people in 1 flat in London.
When ~£13k per annum can support 2 people, £40k is a very high standard of living for 2 people.
If you're fine living in Zone 2 or 3 ( the grey concentric circles on this map: http://www.tfl.gov.uk/assets/downloads/oyster-rail-services-... ).
Then rent in London: Shared house/apt = ~£650 per month, private apt = ~£1.2k per month. Deals are to be had if you can wait for them and do the legwork... those are the averages my friends pay.
Now... on a £40k salary, the PAYE tax calculator here: http://www.listentotaxman.com/ shows that you'll be taking home £2.5k per month.
You can definitely afford London rent on £40k.