People's assumptions about London are so out of whack.
I moved down here from Manchester for a job on 22k, found a flat in Zone 2 that is 4th floor, wooden floors, balcony which you can see the shard and the gherkin from, tube stop outside the door and less than 15 minutes to Oxford Circus.
It's not hard, I'm not talking about budgeting every last penny, living in London is only slightly more expensive than Leeds and Manchester in my experience, as long as you can budget just the slightest little bit.
I'd be interested to know where you're living. I certainly couldn't get by on 21k when I first moved here and I still find it tough earning significantly more than that now. Paying £1,400 pcm in rent hurts a lot.
Unless you're living in one of the rougher areas, living in this city ain't cheap and it's a bloody sight more expensive than the rest of the country.