I think different kerbside recycling schemes have different rules, but mine seem okay.
I tend to take them home with me, make sure that they're absolutely empty (leave the stove valve open for a few minutes with the can attached) and then push a sharp thing through the side of them and bung 'em in our other recycling. Once there's an obvious hole in the can, they're clearly not dangerous any more.
Most recycling schemes will take empty aerosol cans, which are also usually full of residual amounts of inflammable gas, so LP canisters ought not to be an issue. If it is, take 'em to your local tip - they usually have a special section for such things.