Here are the latest posting dates for UK santas, from Royal Mail website.

Airmail
Western Europe - 15 Dec
Eastern Europe - 12 Dec
USA, Canada Japan - 12 Dec
Elsewhere - 8 Dec

Surface mail
Western Europe - 3 Dec (except Cyprus, Greece, Iceland. Malta & Turkey)
Eastern Europe - 26 Nov (including Cyprus, Greece, Iceland. Malta & Turkey)
Elsewhere - you've already missed it. Last date for USA was yesterday