Latest recommended posting dates are as follows:

International Standard (Airmail)

Asia, Cyprus, Far East, Japan 7 December
Caribbean, Central & South America 8 December
Australia, New Zealand, Greece 10 December
Czech Republic, Germany, Italy, Poland 14 December
Canada, Finland, Sweden, USA 15 December
Austria, Iceland, Ireland, Portugal, Spain 16 December
France 17 December
Belgium, Denmark, Luxembourg, Netherlands, Norway, Slovakia, Switzerland 18 December

UK Inland

2nd Class & Royal Mail signed for 19 December
1st Class & Royal Mail signed for 21 December
Special Delivery Guaranteed 23 December

Need more?  See the Royal Mail website.