OECD figures indicate that UK per capita spending on healthcare was near the OECD average, and was less than that of France, Germany, Belgium, Switzerland, Ireland, Canada, the Netherlands, and of course the US. This suggests that the NHS is actually relatively good value for money. Also, it suggests that caution should be taken when comparing the NHS with, for instance, the Netherlands.
The OECD predicts that the increase in funding needed to sustain the NHS through to 2060 would be 6% of the UK's Gross Domestic Product (GDP), taking the spend from the current 7% of GDP up to around 13% of GDP in 2060. Considering that the US healthcare spend is already at 18% of GDP, it is difficult to argue that this is unaffordable.