Copyright in the text of the letter remains at all times with the writer, even after it's been delivered.
The paper and envelope will belong to the sender until such time as the letter is in the hands of the recipient.
However that doesn't mean that a sender, who posted the letter in error, can demand it's return from the postman who empties the box. The letter is fully under the 'control' (rather than 'ownership') of Royal Mail between posting and delivery.