What?
This is a metal.
Where?
This should be brought to a civic amenity site where the keys can be recycled along with other scrap metal. Check the waste services locator on this site.
How?
Remove any plastic tags or key rings before depositing your keys in the mixed metal skips.
General Comment / Tip
Sell them!
If you have a large collection of keys to dispose of, try contacting a local scrap metal merchant.