So a post based on "who you know".
Most grad jobs in London will top out at £30k.
http://www.technojobs.co.uk/search.phtml/graduate-developer/...
Those that are higher are not real "junior" jobs - there's one that advertises "up to £50k", but then asks for 3-5 years experience. 5 years good experience puts you at the top end of mid-level.
There are a few finance jobs, that are looking for junior quant. developers. Yes, they are advertising above £30k. But they don't make up the bulk of the jobs on offer.
(And yes, I have a lot of anecdotal evidence too, for outside and inside London, having spent my early years working for a company with offices in Bath/Bristol/Reading, and the obligatory London "HQ").
All of the people that I know who work in London (not just in banks) are earning at least £35k+, almost always more than that. These are newly graduated developers, with lots of room for advancement.
Conversely I know very few developers outside of London who are earning close to £35k, outside of London its much more likely to be research (ARM etc) and they tend to pay ~£28k