The Royal Mail recommend the following dates as the latest that mail can be posted from the UK for it to be delivered before Christmas.
3 December - Asia, Far East, New Zealand
4 December - Australia
5 December - Africa, Caribbean, Central & South America, Middle East
8 December - Cyprus, Eastern Europe, Greece
9 December - Canada, Poland
12 December - USA.
13 December - Western Europe (but not Greece & Poland. The last date for these was earlier)
18 December - UK 2nd Class
20 December - UK 1st Class
23 December - UK Special Delivery
Shipments after any of the above dates may still be possible on more expensive couriers