Christmas is probably the busiest time of the year for the UK mail.
So in order to send everything you want on time and have it received before the 25th you need to know when are the last dates you can do it.

December 18: 2nd Class and Royal Mail Signed For 2nd Class
December 20: 1st Class, Royal Mail Signed For 1st Class, Royal Mail Tracked 48
December 21: Royal Mail Tracked 24
December 22: Royal Mail Special Delivery Guaranteed

Are you writing to Santa this year? If you are than you definitely would want a reply. The last day you can do that is December 7th.

How do you write to Santa?
You need to send it to one of the following:
• Santa/Father Christmas
• Santa’s Grotto
• Reindeerland
• XM4 5HQ
And make sure you include:
• Your name
• Your address
• A stamped addressed envelope

Merry Christmas!