I don't want to become a "cleaning up this world"-bot. I have my own goals and aims in life, and they are distinct from the goal of "producing as much positive utility for humanity" as possible. I'd rather spend £99 out of every £100 on myself than give it to a random poor person in the third world, because I am more important than s/he is (more important in the subjective, antirealist sense).

Hey, that's fine. You certainly don't have to try to justify your basic utility function. But for people who want to do more to help the rest of the world (even if we prioritize ourselves first), it can be hard just to get ourselves to act rationally in pursuit of this goal. That's the issue at hand.