Creative Review has a great article on the history of the tradition of children writing letters to Father Christmas. Peppered with postal illustrations from the Royal Mail, it tells the story of how it became official postal policy in the U.K. to respond to any child who wrote a letter to Santa as long as they had included a return address. It gives the highlights of this talk given by Ashely March at the British Postal Museum and Archive.
"By the early 1900s the London GPO were receiving thousands of letters addressed to Father Christmas which were dealt with by the ‘returned letter office’ in Mount Pleasant. While the Post Office was obliged to forward all letters addressed to places in other countries – usually Greenland (Denmark) – many of the letters were technically ‘undeliverable’ as they bore addresses, or instructions, that didn’t exist: ‘To Santa Clause, 100 Skies High’; ‘Send this to Dear Santa Claus. He lives in the moon’; and ‘Leave at town nearest to North Pole’ are three examples March mentioned. By the 1920s, unsure of how to deal with the increasing deluge of Christmas mail from children, it became apparent that the Post Office’s idea of returning the letters discretely to the sender’s parents would be problematic – and was hardly in the spirit of the season. But what to do?"
In 1962, the French postal service announced that they were sending postcard replies to those who sent letters to Père Noël, and Britian was obliged to come up with an answer of their own. Denmark had a system where they would respond with a letter, but did charge a small fee. "...it was decided that the British model would emulate the French system and also what had been unveiled in New Zealand whereby a postcard – complete with message – would be sent back to the child without charge on provision of a return address. The case put to the Postmaster General was that, essentially, this was a good Christmas gesture and it would enhance the image of the Post Office. The PO’s one caveat was to not actively encourage the letter-writing, for fear of putting excess strain on the postal system at that time of year." And they are doing it still today. In 2014, they estimated that they responded to 600,000 letters.
I had no idea that was an official duty of the Royal Mail. They've done a good job of not "actively encouraging" it, I suppose. Here, kids must be sure to send their letter to the proper office or charitable organization if they want a response (or, say Grandma and Grandpa's house because they have the hook-up).