I’d call the venue and explain that you’ve been put on a low carb diet by your doctor since submitting the menu and could you work with them to make a few substitutions in your meal?
You might feel a little wobbly when your bs rises to a level you're not used to anymore. But I think/hope that's about it. Should people have a break to smoke or something between courses, (if that's still a thing these days) you might take a tiny brisk walk or walk some stairs or something to burn some carbs off. Main thing is to enjoy the meal and not feel bad about having it. If you're going to have a cheat day, it should be worth it. (Just my opinion, all this. Others might not agree).
If I felt I wanted to do something about it, I would call the venue and ask if they could confirm what you had chosen. My only concern would be if they reeeived all the rerturns, then just totted up how many prawn thingies, how many pasta and so on, and will cater for those totals, expecting attendees to eat what they ordered.
If they arre able to identify your choices and you are not prepared to make the best of them, you could tell them you've had to change the way you eat, for medical reasons and ask if there culd be any potential to alter your choices, and go from there.