I recently used Meguiar's Headlight Restoration Kit and must say that I am impressed with the results. It remains to be seen how long it lasts, but reviews on the internet are good. My ~11 year old 335i headlight lenses came out looking like new after I sprayed on the sealant. My lenses were getting pretty bad and I also used 1000, 1500, and 2500 grit wet-and-dry sandpaper by hand to remove the yellowing and get a smooth surface before using the kit. It only took a couple of minutes with each grit to get them cleaned of all yellowing and smooth. The Meguiar's spray on coating restored the finish to clear as glass.
As per the directions, you will need to tape off the body panels around the headlights and also apply paper or plastic several inches beyond the tape to prevent overspray from the headlight coating getting on your paint.
My before and after results are similar to the photos in the Amazon reviews below.
https://www.amazon.com/gp/product/B01M75GUJB