I think you have been unreasonable OP, sorry.
Your parents sound like they have done their best over the years to come out and see you. Something you have been unable to do in return, so the onus for visiting, both financially and in terms of cost to time and health, has fallen solely on your parents.
And now their own health seems to have stopped them from being able to make a long-haul flight to Australia quite so often, they have generously offered to pay for their entire family to have a trip of a lifetime to Florida and because it doesn't suit you, even though you do have good reasons for that, you are complaining because "they think that they have every right to go where they want and pay for whoever they want."
Which they do, they absolutely do, and that doesn't make them unreasonable. And I think it's that sentence from you that has lost you a lot of sympathy you might otherwise have had on here. They are doing a kind, generous thing and they have every right to do it even if it doesn't suit what you would like them to do.
I understand that you are hurt and would like all the family to come to you, but that would be a greater expense and a longer flight for everyone else.
I might be wrong but a quick google has just said that UK to Florida is 9 hours while UK to Thailand is 17, so you are asking everyone else to double their flying time to cut yours down. Again, I understand you have good reasons, but it's not you organising and paying for this and you want their eleven people to do this rather than your six people.
"They also feel that I chose to emigrate so they shouldn't be penalised for that."
Unfortunately this is true. We have relatives recently emigrated to Australia. There are six of them, the children aged between six and eleven, so it won't be cheap or easy for them to fly back.
So far the only relatives who has been able to afford the time and money to visit them is single and childless, for the rest of us it's looking like an impossible dream at the moment.
They made their decision and they don't regret it, but the onus should not be on the rest of the family to sacrifice their own dreams and plans in order to visit. They understood that when they moved their children away from grandparents, aunts and cousins and accepting that has been hard and sad for everyone.
They made you the same offer as they made everyone else. The fact that your location is further away is down to your own decision and no, they shouldn't be penalised for it. You have declined, which is absolutely your right if you feel it's best for your children but they have continued with their one-off long-term dream, which is absolutely their right.
YANBU to feel upset that you won't see them this time, but YABU to be upset with them for continuing with a plan that they wanted to include you in, even though you have good reasons for feeling unable to take up the offer.
Your DP and his family are being unreasonable and rude to make critical comments about how often your family visit. The next time they start that you would not be unreasonable to tell them to piss off.
I'm glad you seem to have made peace with your feelings about the Florida holiday.