As a general indication, the average software developer in the UK earns about 45k$, while the average in the US is more like 60k$ (source: payscale). The averages for London and Silicon Valley will both be higher than that of course.

Healthcare insurance is more expensive in the US, but this is compensated by much lower taxes.

You're definitely right about holidays and vacation days, you'll have more of these in the UK. At the same time, even when you value a vacation day at 300$ (much more than either average UK or US dev earns in a day) and suppose our average UK dev has 20 more vacation days a year (he probably doesn't), that's still only 6k out of a 15k difference.

I agree that being near your family and friends can be worth a lot more than 15k$.