Serious question: why do you think you're underpaid? Why do you feel you should be making more. The market has said that you're currently worth/getting paid X. So... why think you're Y?
They're very likely in the UK.
They are very likely getting underpaid for dev work, unless they're a contractor. Our salaries are definitely lower here than in the states.
Has it? Seriously, who is this all powerful "market" that one can go ask and get an exact answer? What does "market" have to say about employers simply giving anemic raises, if they give any at all, to employees they believe will have trouble finding new employment?
Someone's current wage is not correlated with the market wages for their current position.
Its often the case that nurses and teachers are considered underpaid as well. While bankers are overpaid. Would you agree with that?