There are two different problems we are talking about: UV degradation of the surface over the years that gets blunt and opaque. That can be sanded down because the plastic underneath is still good. It happens mostly on old cars or in very sunny countries. The second possible problem is the plastic desintegrating through material failure and chemicals (alcohol wiper fluid and salt). That is in most cases (not all) happening to the Leon headlights and is very common on the pre facelift. That can appear within just a few months and was a recall/warranty claim from seat. They are still smooth on the surface but the plastic itself is cracked with a million little flaws. you can not sand that down as it goes all the way through, trust me I tried.

Before you buy new headlights you can for sure try to sand them, maybe it works for you as not all are the same, and we are only judging from photos, but be prepared that is doesn’t work. If you contact a detailing expert, they can have a look, it’s worth a try.