Interesting article in the Guardian questioning the research,the queues,food complaints and the fact that spending money on boosting attainment in other ways would have a far bigger impact- but then that wouldn't buy votes would it.[cynic emotion]
The cost is going to be far bigger when you factor in the cost of building new kitchens,extra equipment for those that have them.Personally unless they massively increase / train dinner staff too(another expense) a lot of all this money will simply end up in the swill bins.
Question- what are you going to do with the kids that never have or like veg?Are you going to make them choose it and remove desert until they've eaten it(can just see the steaming parents now)? If they don't choose it are they just going to have a plate with just a piece of meat and two potatoes ie go hungry?At the mo they give them bread if they don't want the veg and carb and they fill up on the sugary dessert.
I'll wager the schools in the study had extra staff helping choices and sitting with reluctant veg eaters and how long would that last in the real world?