The last posting date for first class letters in the UK which are intended to get there before Christmas is Thursday December 20th. Perhaps extra time should be allowed for letters sent to the Isles of Scilly or the Hebrides.
We keep ourselves aware of collection times from all our nearest post boxes. Our nearest box actually has a morning collection time and we post there at night if we have missed the 5 pm collection time from the box at the nearest supermarket.