One of the main reasons that the average per capita income in the UK is comparatively low is because there are huge numbers of people either not working at all or working part time for extremely low wages. The plain fact is that the UK does not generate enough wealth to sustain an above average lifestyle for all the people who are here. In short, there are too many people here (which is one of the main reasons why there is a housing shortage, a healthcare crisis and a shortage of suitable education places - see some of may answers on these topics).
But that’s not a very palatable truth, it seems.