Why is income tax so high? (UK) Granted, I expect this question could be easily answered, but I cannot find a definitive reason as to why income tax is pinned at such a high rate in the United Kingdom.

I'm not much of an economist, but I do understand the function of national insurance, and I do understand the function of a tax to fund the various essential organs of the state (Monarchy, Government, Parliament, Intelligence Services,and Defense and Protection forces). However, I don't get why, if I earn around 40k (Sterling), a large portion, around 15% (which is around 5-6k), is taken for income tax.
In the UK, government spending is 38.5 per cent of GDP.  To oversimplify, that means that for every £1 spent on private goods, 63p needs to be raised in taxes.  At present, the UK raises a little over a quarter of that from income tax.  One of the reasons income tax is a significant component is that it is progressive, unlike other major sources such as VAT and excise duties.

It is also worth mentioning that the categories of government expenditure you mention are relatively small.  Half of all government spending goes on pensions, healthcare and education.  The categories you mention are marginal by comparison.