I received this reply from the UK a little while ago. I had written to King Charles for Christmas. Normally I write to couple's together, but since Charles, and Camilla have a practice of sending out separate replies I wrote to them separately. You can find Camilla's reply here.
The reply was a folded card. The front of the card features King Charles's Cypher embossed in gold. The back of the card features a photo credit for the inside photo. The photo is credited to the Press Association, and was taken by Jane Barlow.
Inside is a photo and message. The photo is from August 2023, when King Charles III inspected the Balaklava Company, 5th Battalion, The Royal Regiment of Scotland, at the gates of Balmoral Castle.
I am touched to received your kind and thoughtful Christmas message. I hope that you too had a very happy and peaceful Christmas and I send you a special thanks for your wonderfully kind support during what has been quite a memorable year!
This card brings you my warmest good wishes for the new year.
Charles R