Maybe the way to answer your dilemma @WhereDidTheYearsGo is to ask yourself some questions.
1 If you had £1M in savings, would you pay for / towards the wedding?
2 If your DD had only lived with this man for a year or two, would you contribute to their wedding?
3 If they were saddled with a very high mortgage, in a very pricy part of the country, and couldn't afford much of a wedding, would you contribute?
4 If they had gone to a state school BUT your finances were exactly the same now (few savings, I assume) would you pay for anything?
5 Why is a holiday(s) more important to you than a family wedding which you could pay something towards to help your children?
6 Is this a matter of principle ( I've paid for their education, so that's enough) or is it genuinely that you don't have much in savings and it really is either/ or?
FWIW many of our friends are handing over early inheritances to their DCs and we've done the same - passed on inheritances from inlaws directly to the children towards property. House prices in my area are through the roof. I don't know about where you live, but maybe the conversation needs to be along the lines of 'we can give you £x, do you want to reduce your mortgage or pay for the wedding?'