Duvets are the best of bedding for those who are actually on the streets, since they're bulky to carry around and can take days to dry out if they get wet. (Street sleepers prefer sleeping bags, that can be easily rolled up during the day). Some night shelters for the homeless might be able to make use of them though. Try a phone call to Barnabus (0161 237 3223) or Mustard Tree (0161 228 7331) to see if they've got a use for your duvet.
Alternatively, animal shelters are often desperate for things like duvets. Try Millstream Animal Shelter (0161 683 4194)or The Heatons Animal Rescue Group (0161 975 0784).