Yeah, having the whole of the UK being one colour is pretty useless, given the huge disparity in wages and cost of living between central London and the rest of the UK.
I wonder how that skews the figures. Are folk in NI paid more in line with GB or with Éire? I'm sure currency comes into play too, what with Éire using the euro.
I suspect Ireland also has the same problem, with the higher salaries being Dublin only.