[Uther sends it along with Morgana after her visit to give to Arthur. It's not meant to be a way to reach out to Arthur, make Arthur less angry or hurt or anything like that. Just something that Uther feels is necessary to give. He has no wish to cut all connection - he just doesn't know what to do.

Whenever, or if, Arthur chooses to unfold it, he will read:]

Arthur,

Do no forget that you matter the world to me.