http://www.do-it.org.uk/
This is a great site for volunteering opportunities It links you with areas you may be interested in, in towns you want to work. Some of the listings may be out of date but ring to check, there's no harm.
Just to say, the school thing (God some of you do like to attack dont you!) To volunteer in a school, primary or senior, all you need is a CRB clearance, which the school will probably sort out for you.