Is it really a sudden drop? Says who? My impression was the opposite. There might be a structural break at the M25, but I'm pretty sure distance to london and real salaries will correlate significantly. Anecdotally, I definitely command a higher wage in Oxford than I do in Liverpool.
More anecdotes.. I'm starting on double what I'd start on outside of London, and although my accomodation will cost me double what it would back home, everything else will not be that much more expensive. I easily expect the increased cost of living to be more than offset by my salary weighting. Of course, this isn't the full extent as President Ben touched on. I won't, but most of my friends will be on 5x a 'northern' graduate salary in 5 years, and that will definitely cover their increased costs of living! When trying to figure out how good a salary is, pay a lot of attention to whether it increases at inflation, or doubles every two years. Those starting on 22K will one day dream of hitting 30K. Yet many of those jobs that come with a 42K starting salary, also come with an expectation that will be 100K in a few years.