Look off the tube lines but not too far away, you'll get cheaper. Places like, Crouch End (one beds will be cheap as its in the baby belt), Hackney and Dalston.
I pay ?645 a month (admittedly in a shared house but with a garden)in Newington Green, its a decent area, with great bus links to the City yet only about 15 minutes walk to Highbury and Islington tube.
You'll still struggle to get a decent one bedroom place to rent for ?800 a month though, as the rental market is massively over-inflated at the moment as people are not buying hosues.
Watch out using gumtree. When I first moved down I got a decent place off gumtree, but there was a lot of shite.