Don't you get those charity 'please fill a bag' things dropped through your door? I get at least two or three through my letterbox every week. (The record is 11 in one week!). You simply put your items into the bag and leave it on your doorstep for collection.
Otherwise, most charity shops can arrange collection. The British Heart Foundation seems to be particularly well organised when it comes to collecting items. You can find the contact details of your nearest store here:
http://preview.multimap.com/clients/places.cgi ?client=heart
Chris