tldr: despite the flawed stats of the poll, similar skillets are actually paid similarly between London and SV.
It's hard to compare directly between SV and London because they're very different economically. SV is full of huge pure tech companies (the googles, apples, facebooks) that are hugely profitable and HAVE to spend heavily on R&D. Their product is literally technology and so they need 'high end' engineers. This therefore attracts 'high end' engineers from around the world, and the large number of these companies created a bidding war, hence wages rise.
The majority of developer jobs in London (and for that matter everywhere except SV) are at enterprises where they are slogging out backend code - think projects to integrate internal systems. Most of these roles aren't particularly 'high end' and don't need guru 99 percentile programmers, and hence they can pay less. Most of these companies see tech as a necessary evil and not a competitive advantage.
For these enterprise roles, base salary is also meaningless as nearly everyone contracts. A typical 'senior java dev' that pays £60k permanent, would contract for £500/day which is the equivalent of £115k [1]. Rates upto £700 are common, and £1000 isn't unheard-of for rare skills in vogue. Junior programmers should expect £350+
So comparing like-for-like skill set, London programmers are actually paid pretty well - There are many making the equivalent of $150k+ who wouldn't even pass a phone screen for Google. Now London has a few 'high end' roles that pay very well, the majority of which are in finance, with notable exceptions being the London engineering offices of big SV tech companies. Don't be mislead that all of finance is high paying, only the front office roles at the 'computationally intensive' desks are. There are also 'high end' but relatively low paying roles at game companies, F1 and GCHQ.
Another factor is the financial crisis, which has hit London severely. Since 2008 a huge number of jobs have gone, and wages have been essentially flat. If you include the fact the the GBP:USD rate went from $2.10 to under $1.4 at the lowest [2], UK wages actually declined 40% in USD equivalent. This is in stark contrast to SV which in the same period saw huge growth.
The funny thing is that almost the opposite was happening just a few years ago. I graduated in 2006, and London was in the middle of a huge boom. I was at Imperial College, and all the major firms would come to campus and have big flashy recruitment drives. There just weren't enough quality engineers and so starting packages for fresh graduates were literally twice what ANYONE else was paying. Also many of these roles were extremely interesting, working on real problems with cutting edge tech and brilliant people, and the firms had enough money to invest in training, extravagant grad programs etc. This has changed quite a bit now!
[1] http://www.contractorcalculator.co.uk/ContractorFinancialPro...
[2] http://finance.yahoo.com/q/bc?s=GBPUSD=X&t=5y&l=on&#...