Wealth and health inequality
In general in developed countries, high levels of wealth inequality lead to high levels of health inequality. This manifests itself in a huge number of ways. For example, overall rates of stillbirth in the UK are higher than in a dozen comparable countries. International comparisons show that life expectancy is inversely correlated with income inequality: life expectancy in the UK is lower than many countries where the share of national income taken by the richest 1% is lower than in the UK (such as Japan, Spain, Norway, France and the Netherlands).
The authors of The Spirit Level combined a range of indicators (life expectancy, infant mortality obesity, teenage births, homicides and so on) into an 'index of health and social problems', and found a strong positive relationship between high scores on this index and high levels of income inequality across a large number of developed countries, with the UK scoring near the top on both axes. A similar relationship was found between the UNICEF index of child wellbeing and income inequality (although in the other direction, since wellbeing is a positive measure). The UK scored at the bottom of this table in 2007 (the 2020 update showed the UK scoring 27th out of 38 rich countries).
The lesson that the UK can learn from other countries, then, is simple. Building a fairer society, in which income inequality is lower than its current very high levels will improve health and wellbeing and will reduce the prevalence and severity of a wide range of health and social problems across society, but with an especially large effect on the most disadvantaged.