I try and work on an 80/20 rule where 80% of our food is free from UPF. I have two DDs aged 7 and 4. I don't worry about their school lunches for example if food at home is mostly decent.
For breakfast we tend to have either plain greek yoghurt with fruit and honey, porridge or something on toast (Jason's sourdough sold at Tesco and Sainsbury's is a reasonable brand.) There are lots of 100% nut peanut butters out there so that's a staple, or honey, or just butter, or you can get jam that doesn't have UPFs.
Snacks are fruit, plain yoghurt, crudités etc. A lot of bagged popcorn is fine and not UPF. M&S also do a good date and treacle loaf that I give DDs for something sweet. All butter shortbread is often fine for biscuit options.
Lunches are things on toast. Eggs, avocado, hummus, tomatoes, mushrooms etc. Or wraps instead of sandwiches. Crosta&Mollica wraps are just flour, oil, salt, water and really nice. We have parma ham instead of other sliced ham. Serrano is also normally just pork leg and salt. Cheese is usually fine.
Dinners I focus on homemade as much as possible but there's an increasing amount of stuff out there that doesn't have UPF. E.g. the Heinz pasta sauces are normally fine so that with a bag of pasta and some cheese and veg is a quick dinner. McCain Naked oven chips are just potatoes and oil if you want an option.
Standard dinners for us are: chilli and rice, fajitas made with the Crosta&Mollica wraps, pasta dishes, jacket potatoes with tuna, casseroles, risotto, homemade curries etc. I batch cook some things like chilli and curry to make life easier another day.
There's a great Facebook group/Instagram page called Go UPF free or something like that and they share products they find in UK supermarkets.