I've been doing RSPB volunteering weekly since November and it's great! The warden is a great guy and we are constantly making the reserve better (although, I wish we did some more aesthetic work). I've been trying to get some more Forestry Commission work under my belt but they're always working in difficult to reach locations, and only do volunteering once or twice a month. I've also tried TCV, but their work is almost entirely litter-picking. I recently did a week-long volunteering excursion in Glen Affric in the highlands and it was fantastic, although from the way the focaliser made it sound, the weeks usually aren't as fun as the one we had - definitely my best volunteering experience though.