[ig1]

There's not actually that much difference, once you take into account factors like holiday allowance (in the UK you typically get 25 days vs 10 in the the US) the difference is only about 10% - well with the yearly variance of the exchange rate.

in 2008 when the GBPUSD rate was around 2 (versus the 1.6 it is now), salaries in the UK were substantially higher than those in the US.

It's mostly down to currency fluctuations more than any real perception of the worth of software developers.