I wonder if his selective hearing stems from being from a wealthy family? And that he therefore applies a different meaning to the word 'budget' than you do.
To you, 'budget' is an amount of money that you save/set aside for a particular purpose. A physical amount of cash (albeit in an account). When you have spent that physical cash, there is no more.
Perhaps to wealthy-background DH, "budget" means 'planned expenditure'. So if he budgets for something, that's what he plans to spend - but his actual spending is not restricted to his planned spending. In his wealthy-background head, more can be found if he wants to. You said "he can afford to do a lot of things if he wants to that I can't" - I wonder if this is at the root of this? "If he wants to", he has/finds the money, and he presumes that everyone else can. He doesn't see money as a finite resource that can run out whether you want it or not.
So, when you "asked DH if he minded me foregoing our holiday for this as I couldn't afford to do both and he agreed" - he didn't actually make the connection from his fuzzy 'planned spending' to your definite 'actual spending'. Somehow, he thought you could move money around, draw on your savings, wizard it up somehow.
"I think he'd like me to put it on a credit card."
Which would be fine if the bill was paid in full next month, but it's clear you wouldn't be able to do that - so it's a very bad idea. I wouldn't.
"I have told him several times I can no longer afford a holiday this year. He seems to not hear it and continue to look at holidays and show me them."
He'd be getting the rough end of my tongue for that. You've said you can't afford it, continuing to press you to spend money you don't have is unthinking, uncaring, and - makes him look a little bit thick. To not be able to see that few people have bottomless funds at their disposal, to be spent on a whim, is really quite self-absorbed and unobservant.
I'd give it one last go at pointing out that you cannot afford a holiday, and that you told him that earlier this year.
'We talked about this when I was deciding whether or not to do [whatever], doing [whatever] meant I wouldn't be able to afford a holiday this year. I don't understand why you keep showing me holidays I won't be going on.'
Paint him into a corner. Make him spell it out exactly how he expects you to afford this holiday. Then shoot his unworkable ideas down. (No, I don't have £1500 I already spent it on [whatever], no a credit card would just make it more expensive and I still don't have the money etc. etc.)
He can go on holiday with friends, as he has done before. Yes, he wants to go with you - but he can't. As you told him earlier this year.
I really would be giving him side-eye for his lack of financial intelligence, and for trying to pressurise me into debt.