That 'huge sum' is a tiny amount of our GDP and would make very little difference if it were spent within the UK. 0.7% of our Gross National Income. It wouldn't make a ripple in our NHS budget.
I'm sure nobody would be jumping for joy if their annual pay increase was 0.7%. We all complain about the interest on our savings being at that level.
That 0.7% hasn't come out of thin air - it is the target set by the United Nations for all developed countries since 1970 and Norway, Sweden, Luxembourg and Denmark all give a much larger per centage of the GNI. In 2015 the 0.7% was enshrined in to law in the UK.
We didn't hit the 0.7% target until 2013.
The UK is very clever though. We benefit from the foreign aid donations by doing deals - we give you aid if you give us orders, that sort of thing. We are not the world's second largest arms exporter by accident, as an example.