Normally with extinguishers you need to carry out an 'Environmental Disposal'.
The following have facilities for third parties:
Chubb
Thomas Glover and fire trade customers of theirs (goes to Chubbs ERC in the West Mids)
Express Fire
PJ Fire
The top two (or one as it's the same place) have full facilities for the correct recovery& disposal of the media as well as scrapping cylinders, the others have arrangements with scrap dealers but am not sure about the recovery of the media (they don't dump it at their premises though so assume the scrapper deals with that)
For environmental disposals there is normally a charge.
If you just have empty shells, especially if holed, weigh them in. I took a load to a general metals dealer, separated them into aluminium & steel for separate weighing (different scrap value) and got money back for them. I just ensured that they were all empty and had head caps & valves removed and threads damaged to prevent reuse.