I think you just need to admit defeat here, that you have a different image of what things should be, and as its not your party, your image really has no bearing on the day at all.
You would be happy with an amazon balloon display, with a buffet, a local DJ, a DIY do with everyone chipping in.
Your son and his partner wanted an events company run party, with more upmarket food, entertainment and decoration.
Your son didn't ask you for money (maybe understanding that you would offer alternatives such as the above instead), and didn't need your hands as they paid enough so they could have a relaxing day in the run up instead.
In my view, no thank you is needed to you, because they didn't use your time, money or labour. It might not make you happy, but you didn't.
I think you need to brace yourself now that this is probably indicative of the type of wedding that they are going to have- splash some cash and pay for it all to be done. Thats absolutely fine, it will be a lovely day, and above all, what they want the day to be. It's not snubbing you to not want the same as you. If you are providing cash, or paying for a certain element, then yes I would expect a thank you then.
As an aside, if there is a big age gap, then I don't necessarily think giving the same amount of cash as a gift is the right thing to do.
What did £75 buy when your eldest got married? Did it maybe pay for a meal out, with a bottle of wine for them to share?
Would it buy the same now?
Equality and fairness aren't about treating everyone exactly the same, and £75 will go a lot less further now than it did 10-15 years ago.