I had the oppertunity of taking a Job in London recently, I was asking on these boards for advice, my Salary would have been 27-28K, but I have expensive tastes and I'm not brilliant at managing money.
In the end something local came up at my company and the terms and conditions were much better with regards to rest days and holidays I'm now on about 25K, as the cost of living in the East Midlands is much lower I'm staying put for the moment. But as I get free travel and I have lots of friends in the South East I still find myself in and around london 3 times a month minimum.
As I understand it the golden rule of letting agents is that no more than a third of your income should be spent on putting a roof over your head. So in order to rent my own place in London I'd want to be on £30k minimum I reckon.
As my GF is down South it may yet happen, but I think part of her is tempted to move up here.