Not many charities still collect old postage stamps as a way of raising funds but one that does is the Retired Greyhound Trust.
So, if you can bundle up all your old stamps (any used postage stamp will do, UK or other) and put them in a suitably stamped envelope or jiffy bag addressed to:-
RGT Stamp Appeal, Park House, Park Terrace, Worcester Park, KT4 7JZ. UK
You can be sure they will be put to a good cause, helping retired former racing greyhounds.
Of course there are other ways to help the RGT, as you will find if you follow the link to their site. In particular, their shop sells some great stuff - including food for your retired greyhound!