I've booked in on the advice of my DP. He clearly didn't think the kids would be bothered.
You're in a partnership though. He might think that, but there's plenty of evidence on this thread that he could be horrendously wrong. Surely part of being in a relationship is calling your partner on something that they might have got wrong especially when it's important. TBF the kids might not think it will bother them, right now, but who knows how they will feel once it's a done deal.
You're betting all of £600 (let's call it that, though, like others, I can see it's easy enough to reduce that cost) on the fact that he's right. If he's right and you 'lose' that £600 because it turns out that the kids weren't that fussed after all, well, shucks. I assume £600 isn't life changing to you if you can book and pay for a destination wedding. Heck, bung it on a 0% credit card and pay it back over a year.
If he's wrong, and you spend £600 getting married before you go, then you've invested £600 in maintaining your DP's relationship with his kids. They will be happier, more secure and included. That seems like a huge win to me.
I don't want to pile on you, I was a step-mum very young and I made a lot of mistakes early doors. It's easy to do, but this one is avoidable and it seems so obvious to me that the potential cost of not changing your plans here is far worse than the cost of changing them.
The other thing is that if the kids' mum is not likely to be on board with supporting your choice and reassuring the kids, and I don't think she's obliged to do that at all, then I think thinking the kids will be ok with it, because of the attitudes of everyone around them, is severely flawed. Maybe you think that's on her, but again I'd say, it doesn't matter to the kids. They will just feel bad.
TBH, I struggle to believe two competent adults with any empathy can't see that the potential negatives to a plan like this far outweigh any minor cost/inconvenience that an adjustment in plans would cause.