NCVO (National Council for Voluntary Organisations) (ie organisations who have unpaid directors, even if they have some paid staff) has issued some useful advice for if you're thinking about going out. You can find more about some of the organisations we know about on this website, here:
Some of the organisations we know and love
And here's the general good advice from NCVO:
www.ncvo.org.uk I want to volunteer
Keep us posted on your experience...it's all incredibly useful for us all.