What percentage of GDP (tax) does the UK spend on health?

Short answer = 10.2%

In 2019, the UK spent 10.2 per cent of GDP (your hard earnt taxes) on health, which is similar to the average amount in comparable developed countries.

For 2020 it rose to 12.8 per cent, due to the increased funding allocated to tackling the Covid-19 pandemic. It will now come back down again.

Other resource
There is more information on the King’s Fund website.