I just found this website called Landshare. If you have some land that you'd like to use to grow food, you can use the site to find people to help you prep the land and plant stuff (and vice versa). I'm trying to rope people in to sorting out my garden (i can't see the ground for nettles...). If you live in the Hackney area and you're keen to grow veg, give me a shout!
Check out Landshare