People wouldn’t necessarily know that an envelope is from Kit Harington and Rose Leslie just because a Jon Snow stamp was on the front.
I’m assuming Harington and Leslie didn’t include their names and address on a return address label.
The Royal Mail released a series of limited edition stamps earlier this year bearing the face of a number of Game of Thrones characters, included Daenerys Targaryen, Jaime Lannister, Arya Stark, Cersei Lannister, Tyrion Lannister, Olenna Tyrell, Ned Stark, Tywin Lannister, Sansa Stark, and yes, Jon Snow.
Such an envelope could have been sent from any Game of Thrones fan in the U.K.!