It's very hard to answer, sure there are opportunities for good deals but they're not publicised. OK, I saw a one-bed place in West Hendon, very small, £650 a month, communal swimming pool etc - you'll have to be bussing it or using Thameslink though. And what's the point of a swimming pool (which is probably shit) if you hate the area and feel isolated?
Most places can access TCR in 40 mins, unless yr really out in the sticks - it'll be easier to give you areas to avoid - Angel, Old St, Bethnal Green, Islington. Believe me, if these were available at this price, I'd be living in them now. Also, do you really wanna pay £700 and have a restricted social life? You'd be better off getting a mortgage.
NW London has some cheaper deals around Harlesden, etc, but you'll be living in a bit of a dodgy area, sorry I have to shoot off now, but I'll tell you more tomorrow