Regardless of cost or nutrition, I'd love to see free school meals for all children simply due to the socialisation aspect of eating together. I feel more and more sad about the "fragmentation" of younger people eating separately, some in dining halls, some in specially designated areas for "packed lunches" and some just eating in random places, i.e. the playing fields, form rooms, library, random corners and benches, etc. I think it causes problems with socialisation, loneliness, and even bullying etc.
I can't remember why exactly, but I took packed lunches to secondary school, so ended up lonely at lunchtimes because my friends went to the dining hall for school meals. I remember asking my parents to have school meals, but it was a "no" from them, and I can't remember why. It led to me being awkward and embarrassed when eating formally with others once I left school, i.e. works christmas meals, "dates", etc.
My son has ended up the same. He got into the habit of packed lunches at primary school because he was a "fussy" eater and, to my shame, we facilitated it by giving him the choice. I wish we hadn't. He did the same at secondary school, despite them having a "canteen" style with plenty of food options, he just wanted his packed lunch and ended up eating it in random places, so he, too, missed out on "communal" eating and he was the same at Uni, always getting a sandwich or sausage roll from a shop rather than using the dining halls that the other students used, and he ended up lonely and friendless at Uni!
As for nutrition, I really wouldn't "push" vegetarian only options nor ultra healthy unprocessed foods. I think there's a middle ground of giving options, including unhealthy options to a degree, but I'd draw the line at foods/drinks which contain "dangerous" levels of salt, sugars, etc., i.e. just serve food/drinks with green or amber warning signs, and not red warnings. Give a choice - I think mandating healthy or vegetarian options is a step too far.