My knowledge of what the laws are and when they came in is very rough, but this much I know; it used to be the case that any traveller could volunteer on UK projects (and thereby have a more indepth experience of their visit, as well as contributing to the local community). however, in the last few years, this right got taken away.
I work on a small organic market garden that grows salads, potatoes, onions, tomatoes, fruits. As far as I see it, the food from this garden is the tastiest and healthiest that can be found, far more so than shipped supermarket produce. The opportunity for me to work on the farm is good for my vitamin D andexercise levels. There should be many many more market gardens like this on the outskirts of city. It is the sustainable future of food.
Anyhow. As it is, these farms struggle against the 'economy of scale' that supermarkets operate on. One thing that counteracts this is a network of volunteering (through Help Exchange or Woofing) where people come from all over the world to work for free, for the experience, education, and often the opportunity to practice language.
Ever since the law changed, it became illegal for people on a tourist visa to volunteer. I cannot think of one good reason why this is a good law. I challenge anyone to come up with a reason. Its a stupid law. If people are willing to come from the other side of the world to give their time to good causes, please let them do it.