I WWOOFed at Karuna Bhavan, a Hare Krishna community about 45 minutes outside of Glasgow. Delicious vegetarian food and a great place to learn about Hare Krishnas. I volunteered in January so it was a little cold but I think the spring and summer would be great.
I've also volunteered at Pillars of Hercules organic farm in Fife, Scotland. I'm not sure if they're looking for more help right now but it's a great farm with a little cafe and farm shop attached.
Hope this helps!
Emily