Oof. I have no idea whether the itinerary included the exact locations, but could you research menu's online and check whether they have something suitable for you?
Wait a sec, lemme google...
Oh, oooh....!
https://www.rockfishchips.co.uk/home is this the place? Because if everyone else is having chips, how about you get the prawns, scallops, or grilled mackerel? Looks good in the pictures, and low carb. On top of that, they do mention they are attentive to people's allergies, so if you just order heaps of fish and no fries, I think they'd be fine with that. Or get in touch beforehand and see what you can work out. But I think you won't miss anything, and who knows, you can maybe steal a fry or two off others to satisfy for a bit of a taste.
As for the pasty, is just eating the filling and feeding the rest to the gulls an option? I only ever had pasties that spiked me, but they weren't the real British thing. I don't know where you'll be at, but if all they have is pasties, that might be the way to go. Or share with someone other than the gulls, of course.
Can't help with the cream tea, though I have to say I'd have a really hard time with that myself. (Our wedding reception was an afternoon tea. High tea was listed, but we call all teas high tea here; we're heathens!). Maybe ask for a coffee with cream? Still decadent and filling, gives you something to handle while the rest eats. And a sunset with tea seems more pleasant to me than one with cocktails, but that's personal preference.
Look for work-arounds. And yeah, do involve your daughter in law, if you can. It all sounds like an excellent itinerary if you don't have to watch what you eat. Letting her know you want to participate but need to do it on your own terms might help get the next outting sorted beforehand, because she'll know what to keep an eye out for. (Namely, alternatives at the site!)