I think what most people are missing in these comparisons is that London's tech market is hugely contractor oriented, and so it makes more sense to compare day rates. Finance in London pays typically £500-600 per day for average level frontend and backend work. There's the potential to earn much higher if you're good at what you do.
Most companies I know much rather have permies than contractors.
Pay tends to be quite a bit higher for contractors but without all the perks like pension contributions and leave that a permanent position has. However, as a contractor, you have more control over your life.
As a contractor, it's also easier to get exposure to different tech, rather than be stuck doing the same thing day in and out.
