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.
4 December - Asia, Far East, New Zealand
5 December - Australia
6 December - Africa, Caribbean, Central & South America, Middle East
9 December - Cyprus, Eastern Europe
10 December - Canada, France, Greece, Poland
13 December - USA.
14 December - Western Europe (but not France, Greece & Poland. The last date for these was 10 December)
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