It's got to become a sensible goal, because too many people -- as Helen noted, in critical areas like schools, fire stations, etc -- are being priced out.  My read of it is that a person needs to bring in roughly £15-20k, after income taxes, to live a fairly comfortable -- defined here as having plenty of money to afford basic necessities (a flat or, more likely, flatshare; food; utilities; council tax; etc) and a little bit of spending cash -- young person's kind of life in London, based on the lowest rents I've found being about £400/month (give or take a few tens per week) inside the M25 and using the old 25% of Income rule.  Problem: £20k is a lot of money.  It's roughly the median for a given person in one of the developed countries.  You're, of course, more likely to earn more than that in a major city like London, but probably not that much more.

I, honestly, don't follow the reasoning behind businesses not having moved already.  Most of the recruits from universities are going to be well outside of London.  The rents are going to be cheaper.  The wages paid are going to be lower, given the difference in the cost of living, alone.  It doesn't make sense, economically, to feed everything into it, the City be damned.

Conservatives want live babies so they can raise them to be dead soldiers. - George Carlin