My wife is from the UK and has worked in London and NYC and she makes quite a bit more money in NYC. She has told me there's no way she'd make her salary in London and it would probably be quite a bit less. She doesn't work in software either; she works in finance.
I think part of it is it simply costs a whole lot more to employ someone in the UK. All those social benefits have a cost and companies have to pay them. It's also much harder to fire someone in the UK than the USA where you can be fired on the spot for no reason at all. The benefits the company has to pay out are technically none except for some unemployment insurance for a few months which is quite small all things considered.
We've talked about moving to the UK but our lifestyle would change for the negative we believe due to the low salaries and the very high cost of living anywhere near London. Especially with the beating the Pound has taken recently when we convert our dollars to Pounds there's no way we'd get close to our current income.
To be honest I'm not sure how you folks do it. The UK is expensive and people simply do with less in general. That's not such a bad thing either.