I think the RSPB does. Certainly the Bransby Home of Rest for Horses does. Their address is :- Bransby Home of Rest for Horses, Bransby, Lincoln, LN1 2PH. They ask that the stamps simply be torn round from the envelope.
I think some send them to those stamp companys who sell or give away bags of "unsorted" stamps to get you to look at their approvals and hopefully buy from them. The blurb used to go "that many rare and priceless stamps had been found in these unsorted bags" but in reality it is mainly 1st and 2nd class british stamps that you or I send in.
jan
I used to be on the local Lifeboat committee and we collected used postage stamps. Also my loval butcher collects for the Tunbridge Wells Hospice and the post has a box for them but I don't know where they actually go.
I work at a local infant school and we are always collecting something for charity. We are currently collecting used postage stamps for Guide Dogs for the Blind. May I suggest you enquire at your local schools? They would welcome any contribution, I'm sure.
Failing that I would suggest the following site.