If they're in good condition, mattresses could be sold online, taken to a charity shop or given away on Freecycle (opens in new window) or Freegle (opens in new window). Some local authorities and charities might even collect reusable items from your home. Please note that any furniture containing upholstery needs to have a fire label on it, or it cannot be reused - see Reuse Network (opens in a new window) for more information.
You can take old mattresses to one of north London's reuse and recycling centres (opens in new window) (RRC). We recently started a mattress recycling (opens in new window) trial at some of our sites.