Have been chucking mine/taking them to pharmacy where they get destroyed.
Now I find they can be recycled! This has been around for a while but I didn't know about it until very recently and thought I would share if Asthma UK is ok with that. The site has a list of pharmacies near you which do this plus a list of what you can recycle (most types of inhaler devices eg Ventolin aerosol, Respimat, Symbicort dry powder, Nexthaler etc etc can be recycled with this; spacers and nebulizer stuff can't).
Sadly not all pharmacies do it yet (and some need to train staff better as I went to one - a Boots - claiming they offered It and the counter assistant had no idea - suspect those ones got destroyed despite the scheme being on offer.)