Noticed this on the app . Got some screenshots but unfortunately forgot to check the expiry date (It's one use per day). If someone could find that out and let me know, i'll set it here. Also, please let me know if it state whether it applies to all hot drinks or just coffee.
As usual, this may or may not work by just showing them the image, but there is no unique identifier, so it should't be a problem.
If you have the Sky Perks app, it should be in there.