However much most of us would like to see foreign aid reduced it's very unlikely to happen. First of all we'd really need to change the law (Cameron's) that we will give 0.7% of GDP and I cannot see that happening. None of the parties has a policy of supporting reductions in foreign aid, and some want to spend more. I don't think we could realistically come close to eliminating it anyway as a significant part of it is not aid in the sense that people think of (cash handouts/food for the starving or anyone else, buying Rolls Royces for ruling classes) but is really trade sweeteners/deals, political/military influence, investment in infrastructure that will help us- and when we leave the EU we will want more trade with some of these countries.
But we know money is wasted and some does go in brown envelopes or to support LGBT bands in Africa so the best I can hope for is that a party with a chance of being elected decides to reduce the figure incrementally to say 0.5% over 5 years and then to say 0.3% after 10 years. We probably could not go much lower.