>They do put cornflour in their fruit yogurts, but that seems less egregious than most.

These are UPFs. The point of adding cornflour or starches is to make them thicker and more luxurious in terms of the way they feel in your mouth without straining them like greek yoghurt to reduce water i.e. they're able to do it with less milk protein and fats = it's cheaper. They do this because you need 3-4x as much raw ingredient to do achieve the same result. Adding fruit purees which is the way they add most fruit to the yoghurts is similarly UPF.

There's a scale called NOVA which categorises products based on the amount of processing. Plain Yeo Valley yoghurt is fine and would be classed as NOVA group 1. Their yoghurts would be NOVA group 3 or 4 and so either processed or ultra processed. This isn't to say that all UPFs are bad but it's become so normalised that everything is ultraprocessed that we don't even realise it.

My wife's sister is a food scientist and this is literally her day job at a large American conglomerate with substantial UK operations - try to reduce cost of production by reformulating products while maintaining the same flavour/texture profile.