Bit late here, but we pay £800 pcm for a 1 bed in central-southwest London (Zone 1/Zone 2 borders). The price is EXTREMELY cheap for our area, since the basement below us is similar configuration but smaller square footage and they pay £230/wk (we pay £185) - we think it's because we use a private landlord & not an agent. Private landlords TEND to be a bit cheaper, plus you avoid nasty agency fees.
It's hard to give a percentage, b/c the boy works in sales so his salary ranges widely every month. My share of the rent is about 40% of my net salary, but I just admin temp at the moment, so my income is rather low.
Let's just say I'm very jealous of those who pay 30% or less!! Love London, but my wallet will be happier when we're ready to buy a house somewhere else (still in England, but not urban!).