Any mail addressed to you is not "yours" until it's in your possession.
It's in the possession of Royal Mail.
I don't think it's in control of either the sender or the recipient while it's in transit. It's in the control of RM.
If you sign for something in your name, and the postie hands it to you, it's in your possession. If a letter or jiffy in your name is delivered to your address, it's in your possession.
If something arrives not in your name, don't open it, as it's not addressed to you...leave it near your front door with "Not At This Address" written on it, it's obviously not meant for you, and you were going to drop it back into a post box next time you go out shopping.