I feel sorry for you too, OP, as you must be run ragged and trying to do your best in a situation which must seem unfair.
This may come under one of the unhelpful comments in your mind, but at one point in my long and tortuous (ongoing) separation process with xH2, I sought a second opinion on the legal process from the best family lawyer I could find. Apart from confirming the tack my own solicitor was taking was the best one, she made the following point. It was my responsibility as a parent to do the best I could to manage my relationship with my ex as best I could in the interests of dc. Now that relationship was, and is, enormously difficult and there is lots of unfairness in the way finances have ended up, so this did jar. My instinctive reaction was 'but, but, but...'. In a sense, I think her point was that I had a responsibility to make the separation work for dc so that they maintained a relationship with both parents.
The other thing she said, which is quite hard, was not to be so involved. By which she meant, I think, was let the underlying issues with him go, and focus on dc now.
I think the pertinent questions here would be does your xH know that you can't afford school uniform? As far as I remember from the thread, you agreed to not increasing the maintenance when dc were with you more. Have you gone back and said, actually, this does not work, we need the proper amount? Have you tried saying, actually, my hands are full, I am not well myself, this is the practical help I need with the dc that are yours? Because at the moment you have the dc (no time to yourself) and also no money. You can ascribe all sorts of motives to your xH in paying for this trip, but unless you have had these conversations, you can't actually know that he would not have acted differently if he were in full possession of the facts and had the chance to behave like a responsible parent?
And finally, as I think I have said enough on this thread, my feeling in your situation would be to let it go. Ask your ex for full maintenance and what you need in future, but if the trip issue is upsetting you so much, let it go. It is really not worth it.