Have you bought the wrong size and forgotten to take them back to the shop? The charity Smalls for All takes new, unworn pants in women's and children's sizes and gives them to people who need them in Africa. They must be either a woman's size 8-16 or a children's size for ages 3-15. Send them (still in the packaging) to:
Smalls for All
108 Buchanan Crescent
Eilburn
Livingston
EH54 7EF
Source: http://www.smallsforall.org/get-involved/donate/, confirmed by email January 2018.
Smalls for All also takes unworn or gently worn bras (but knickers and underpants must be brand new).