An air fryer might be worth it. You could probably justify the cost on the amount saved in a single restaurant meal compared with cooking in the accommodation.
But not all ready meals are ultra processed. M&S do a couple of versions that are just protein and veg, so if there's one of them nearby, that could be an option, but they are quite expensive - £6-7 each.
Sometimes you can find ready made curries that aren't mass manufactured, eg on counters in Asian supermarkets or specialist shops, but obviously area dependent, you'd not find these in rural Devon for example.
What about Cook? Their food isn't UPF and some will be microwaveable, but it will take a while to microwave individual meals for a family.
Is there a toaster? A freezer? One of my go to breakfasts (Mumsnet orthorexics look away now) is omelette, baked beans and potato waffles done in the toaster, so you could replace the omelette with microwaved scrambled eggs if there's no hob.
Have a picnic/deli meal so cold food only. Cheese, serrano ham, nice bread, olives, mariated artichokes etc.
But tbh, I think you're making life a bit hard for yourself. It's not possible to totally avoid UPF and some ready meals are quite nice even if you generally avoid ready made food. I'm thinking the nicer/more expensive ones eg Taste the Difference/Finest. Most supermarkets do a ready made paella that's quite nice, also curries and some pasta dishes. As a one off, that's what I'd buy.