I don't think you are BU. It looks like the position between you and your parents has become polarised: hurt on both sides? There seems to be a pattern of stand-offs going on?
So there's you in Oz going 'they're not fair!' and there's them in UK going 'they're not fair!' and the battle lines are drawn. I think at the start of your thread you (both sides?) were entrenched in the 'not fair', and maybe OP you weren't in touch with what you really feel about this.
Bottom line is that it has HURT you that they don't appear to be supportive or care - particularly as they have what looks like twisted the knife by saying 'right that's it - we're taking all the others on holiday and because of the expense we won't be seeing your children for a good while'.
OUCH FOR YOU
The very thing you have wanted from your parents has been thrown in your face, used to wound you? Families can have an uncanny ability to hit where it hurts the most
. Add to this that they seem to be dismissive of the SNs 2 of your children battle with (that hurts). Add to this - and I'm taking my life in my hands here: I'd find it difficult if my parents were well-off and were happy to see me and my family grinding along in need of a bit of help.
Maybe you've been hoping all along that somehow the rejection you - but especially your children - suffered at the beginning (when you had 2 children and your parents 'ignored' them when you were in the UK) was a blip, and you've been hoping for a happy ever after (those of us with fractured families all hope for that imo). This last incident has driven home what appears to be yet more of the same, more rejection - so the hope has been dashed, again
. I should imagine that your siblings accepting the holiday - or, accepting your parents' stand - rubs salt in the wound: won't anyone value you and your family? Won't anyone make a stand and stand up for you? Perhaps this is at the bottom of your outrage: that you don't feel valued by your parents or your family. That hurts.
Just a thought: maybe you moved to Oz to 'punish' your parents for not caring for you and your kids? You may not have done it consciously (if, indeed, you did it) but that motive could have been under the surface somewhere? 'right! If they don't value us then we'll bugger off - serves them right'
And maybe , as a family, you all do the 'punishing' thing: 'Right! you've hurt me so I'M GOING TO HURT YOU MORE' etc. Families/people can do this type of thing, get into the habit, don't realise they're doing it; hurt piles on hurt; punishment and counter-punishment.
That's just a guess, anyway. I don't think UABU to hope that the whole family would want you to be happy for them to be happy (or: wouldn't be happy unless you were happy). I don't think UABU to hope that your parents would not enjoy a once in a lifetime family holiday without all the family . (I personally don't think that just because they're paying for it they can dictate: a mean/exacting giver can leave a bad taste - but that's my opinion, clearly not generally shared on here.)
Somebody said upthread that there has to be a huge backstory to skew the picture today. I'm guessing there is a big backstory and there's been a lot of hurt. You hoped they'd step up to the plate once and for all - maybe they're hoping the same? (I really don't know)
Well done for sending them off with your blessing - that must have taken a lot of courage. Maybe it will disarm the stand-offs by coming at it in the opposite spirit.
(I'm just guessing here and I'm sorry for the very long post)