Sent LOR to Buckingham Palace address. Received a short form letter in response, but still, sorta cool to get a letter from Buckingham Palace. (and sorta what I expected)
Coming from a member nation of the Commonwealth (Australia), it is well known here that no member of the Royal Family sign anything (official books of rememberance etc are the exception) and don't pose for selfies, although the actress formerly known as Meghan Markle has been breaking that tradition of late.
But yes, you did get a reply from the Palace which is really cool - and a nice keepsake. Congrats.
As David says, you won't get any Royals autograph. The only one you'll get is the current Queen/King and you only get these when you get a telegram for your 100th Birthday or your 60th Wedding Anniversary. Luckily I have got 2 as my grandparents have been married almost 70 years and my great grandfather reached 102