Dassie got all the complicated stuff outta the way, as for a little more on the structure of your discussion:
Might have to consider the cost of living over there compared to here, very expensive property/land over there, long working hours. Cosider social, economic, environmental,demographical, legal, technological etc factors. The relative density, land mass/area compared with the UK. With possibly the most important being economic, social, environmental. Try to find a report online detailing the comparative demographics of the two countries for that year. (socio-economic factors). Not to forget your daughter is to evaluate the accuracy of the DATA given. I have suggested alternatives, she should consider the reliability of the data as an indicator in terms of: Accuracy, timeliness(up-to date?), relevance, etc.
Is gross just afer tax or is this after average expenditure?
If the former then the preceeding stuff comes into play if the former then this will be easier to discern.