The whole UFP thing is so wishy washy. It's such a broad category it is essentially meaningless, one of the earlier guardian articles talked about sliced bread being a UPF as it has added vitamins and minerals due to law.
If you closer at the whole topic it just comes across as saying if you are poor and eat food that you can afford you will die earlier. Writing a reminder of this on the food will not help anyone but it will make poor people feel a bit more like shit.
We already have a traffic light system on foods and it is largely ignored, partly because few people have the time and energy but also because you have to look at your diet as a whole and not judge it by individual items. I also think these UPF studies have a bit of a conservative agenda.
Why do people eat more processed food? Because there isn't a homemaker spending hours a day preparing meals as a full time job. Proceesed and convenience foods are massively egalitarian and I think let more people join the workforce.