You would need to be an upper tier waste carrier for the collection and have an environmental permit for the recycling unless you are very low volume (see
https://www.gov.uk/government/uploads/system/uploads/attachment_data/file/298092/LIT_7753_2353c2.pdf)
Whilst a couple of scrap yards do operate extinguisher recycling on a commercial basis the market is pretty sewn up by the two big players Fire Protection Recycling & Chubb who both have massive plants in the West Midlands - FPR also offer a one stop shop as they do batteries, detectors, halon, etc as well.
Also unless you are dealing in scale it's not economic, disposals are around a quid a piece with CO2 actually making you money (or credit) from the CO2 refurbishing companies.
Old extinguishers are sometimes sold 'serviced' by some contractors who want two bites of the cherry, they sell the customer a new extinguisher and their old one ends up on eBay (& I'll eat my hat if it's been extended serviced or overhauled)