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.