Last Royal Mail Christmas posting dates from the Post Office
To find out more, and read the comprehensive list, check out this page on the Post Office website here.
One cool little feature this year is the ability to add the last posting days into your calendar as a handy reminder using their widget. You’ll see it on the page linked above.
This year Christmas day is on a Sunday. The last First Class guaranteed delivery posting date is Wednesday 21st. Although you can send Special Delivery Saturday Guaranteed on Friday 23rd and rest assured it will be delivered on time.
It will be interesting to compare and contrast these last Royal Mail posting days with those of the various couriers and, indeed, Amazon itself. I recall that last year you could order on Amazon, if you were in an Amazon Prime Now zone and a Prime user, up until 9pm on Christmas Eve.