Can you get parcels you've "sent" back from the post office
No, that's interfering with the mail
Once the post office has it, the mail belongs to the addressee until Royal Mail has used its best endeavours to deliver it, after which they may deliver it to the sender if the sender's details are marked on the mail item.
No, that's interfering with the mail
Once the post office has it, the mail belongs to the addressee until Royal Mail has used its best endeavours to deliver it, after which they may deliver it to the sender if the sender's details are marked on the mail item.