I disagree with a lot of that second bit - there’s very little evidence that voters are opposed to foreign aid in “all its forms”; the amount of money raised privately via charities and by states is massive.
The fact that rags like the Mail and that other one devote years to slagging foreign aid off should tell you that they need to demonise it, which means it’s not demonised now.
I do think the West needs to do much more diplomatically, and we certainly need to get ahead of this movement for UN reform (personally I think advocating regional Security Council seats is the way to go there) but sayings it’s down to what Western electorates will tolerate is wrong imho.
By 'foreign aid' I mean funds paid from government coffers to other governments. That's the standard definition of the term. If we're talking Sally Struthers (and apologies if I'm speaking Greek across the Atlantic re: private contributions), that is not foreign aid. We call those funds private contributions to an NGO.
People in the US believe that they alone should decide where their money should go. This sounds appealing on the face of it, unless you understand the problem of public goods. If you like parks, student loans/paid higher education, three percent down payments on a house, food and medicine containing the ingredients on the label, air traffic controllers, highways, bridges, functional financial markets, health care in retirement/for all, not having runs on banks and national defense, then you need a government because private industry will undersupply or not supply all of these things. Foreign aid also falls into this category.
It may be the case that UK voters are more sophisticated than ours. Over here, foreign aid is a dirty word on both sides of the aisle.