While there are certainly companies trying to hire iOS developers at 40k, there are also companies trying to hire iOS developers on 80k+ and iOS contractors on day rates of £500/day.
What you're probably seeing is selection bias. That is the crappy jobs (i.e. low pay) stay on the market far longer than good jobs. Hence you end up thinking that crappy jobs represent a large proportion of the job market than they actually do simply because they stay around longer.
But if I go and search: http://www.technojobs.co.uk/search.phtml?page=2&row_offs...
You will see most senior dev roles advertised with a range, that seems to average out about £45k. That's for the whole country, and London seems no different.
Personal experience with London based recruiters was that despite jobs advertising £60k, the number that the recruiter had from the client was actually lower. When asked about the requirements for getting the top end of the pay scale, "Well, they really only want to pay low 40's", was something I heard time and time again, and caused me to not apply, time and time again.
Crappy jobs? Maybe.
EDIT: Also, I've seen an expensive corporate salary survey (shh, don't tell HR), and whilst London mid-senior people have seen a good bump to their salary in the last 2-3 years, the averages there were approx: junior ~£28k, mid-level ~£36k, senior ~£47k, architect ~£60k, dev lead ~£72k).
As someone working in finance in London, this seems way too low. I'm on 97K and bonus has been 50% for the last 4 years. Contracted before and made more after tax.
I can't believe that startups hope to attract talent with 35K + bonus? But I also doubt you're right about 45K jobs in finance being anything but entry level.
http://www.itjobswatch.co.uk/jobs/london/c++%20developer.do
(me? c++, perl, python, unix, front office, could price an option with Hull open, or an IRS, but not going to try anything like a Vol swap without Quant Assistance. There, another vague datapoint).
That's before you start looking at contractor rates...
http://www.technojobs.co.uk/search.phtml/php/searchfield/loc...
There are a few above 40, but they don't sound mid-level/generic.
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
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").
But to me, they are the outliers, of what is a market that is generally not paying much of a premium above outside London salaries (I knew some architects on £70k, waaaay outside of London, and many seniors on £50k + bonus).
And I didn't want to make the comparison, but yeah, these numbers are terrible compared to what you will find in the major USA cities.
When start-ups, profitable ones, are offering £35k + bonus + stock options (worth £0), for senior developers, in Central London, there's not a talent shortage.
Similarly skilled developers are getting £35k+ & bonus outside of London, where the cost of commuting/living is significantly cheaper, in established companies with full benefits packages, that don't expect them to work 50 hour weeks. Inside London, those devs who are working in finance are getting paid £45k+, big bonus, gym, laundry, etc. thrown in, with the same workload expectation.
Experienced (and good) Senior iOS developers in London are regularly being offered less than £40k.
No surprise many talented staff have left London, for equivalent salaries, less stress and cheaper cost of living (especially as they hit the age where kids appear in the picture).
No surprises many younger devs look at the £25k starting salary in London, where transport and rent will cost them £1000/month, vs. £25k out of London, where it will be more like £500/mo, and decide that London is not worth losing £6k/yr after tax.