I feel you anguish Robbie, we also have faced this awful situation four times in our lives with cockers, most recently in February with George.
We all know this time will come, and you have been blessed with 15 long years with Charley, a good age for a cocker.
Hard as it is I think you know the right decision, from what you have written I think Charley would thank you for helping him to the bridge, a last kindness you can do for him.
Your pain will ease and you will be left with fond memories of a dear friend.
So sorry you are having to face this.